Magnetospheric System Research Articles

Theory and observation of auroral substorms: A magnetohydrodynamic approach

A theory of auroral substorm dynamics is constructed on the basis of MHD wave processes in the ionosphere‐magnetosphere system. The basic view is that the substorm commences in the nightside near‐Earth magnetosphere through a collapse of plasma equilibrium. The collapse releases a significant amount of free energy embedded initially in a collection of compressional waves. It is suggested that substorm dynamics after the collapse are determined by the evolution of these waves. We first investigate the quantitative ramifications of the waves in a two‐dimensional box in the GSM yz cross section of the magnetotail. The model is constructed to allow the study of radiation of substorm wave energy into the solar wind and also encompasses the essential elements of resonant interaction in the plasma sheet boundary layer. The natural boundary condition leading to radiative loss is introduced. It is found that wave radiation into the solar wind can relax the magnetospheric system in less than a hour. The resonant Alfvén modes driven by the normal compressional modes in the box are studied through the construction of proper dispersion equation. By studying the field‐aligned current generated by resonances, we establish the auroral pattern expected to result from the coupling. Following the theoretical study, we examine an auroral substorm observed by the CANOPUS photometer array on February 20, 1990. It is found that, among the testable theoretical predictions, there exists a general agreement with the observations. We did find, however, that electron‐ and proton‐induced aurora oscillate essentially in phase, thus implying a more complicated precipitation process.

One‐dimensional hybrid simulations of current sheets in the quiet magnetotail

Hybrid simulations (kinetic ions, massless fluid electrons) have been performed of the magnetotail current sheet for parameters characteristic of the quiet magnetotail (υD, bn < 1). Here bn = Bn/B0,υD = (cEy/Bn)/υi,Bn is the north / south (z) magnetic field, B0 the asymptotic value of the tailward (x) field and υi the ion thermal speed, where the magnetospheric coordinate system is used. It is shown that the current sheet is more stable than in previously examined cases with high υD and / or large bn. A clear single reversal in Bx persists over the entire simulation and a By component is created, with a slow rate of growth (hundreds of Ωi−1, where Ωi = eB0/miC). This is many minutes in real time. It is suggested that the waves are due to an electromagnetic instability of the cross‐field ion current.

The magnetospheric lobe at geosynchronous orbit

On rare occasions, satellites at geosynchronous altitude enter the magnetospheric lobe, characterized by extremely low ion fluxes between 1 eV and 40 keV and electron fluxes above a few hundred eV. One year of plasma observations from two simultaneously operating spacecraft at synchronous orbit is surveyed for lobe encounters. A total of 34 full encounters and 56 apparent near encounters are identified, corresponding to ∼0.06% of the total observation time. Unlike energetic particle (E>40 keV) dropouts studied earlier, there is a strong tendency for the lobe encounters to occur postmidnight, as late as 07 local time. The two spacecraft encounter the lobe with different rates and in different seasons. These occurrence properties are not simply explicable in terms of the orbital geometry in either the solar magnetic or the geocentric solar magnetospheric coordinate system. A composite coordinate system which previously organized more energetic particle dropouts is somewhat more successful in organizing the lobe encounters, suggesting that solar wind distortion of the magnetic equatorial plane away from the dipole location and toward the antisolar direction may be largely responsible for these dropouts. Our results further suggest that this distortion persists even sunward of the dawn‐dusk terminator. However, a simple dawn‐dusk symmetric distortion does not fully account for all the seasonal and local time asymmetries in the occurrence of the lobe encounters; thus there is probably an additional dawn‐dusk asymmetry in the distorted field. The lobe encounters are strongly associated with magnetospheric activity and tend to occur in association with rare magnetosheath encounters at synchronous orbit. It thus appears that the presence of the lobe at geosynchronous orbit is the result of major, probably asymmetric modifications of the magnetospheric field geometry in times of strong disturbance.

Properties of AE data and bicolored noise

A detailed analysis has been made of 13 periods of 20 days of the AE data. The average correlation dimension is found to be 3.4, and the calculated dimension is found to depend on the magnetospheric activity such that more active periods have smaller dimensions. This apparent correlation dimension does not, however, imply that the magnetospheric system behind the AE data is low‐dimensional. On the contrary, bicolored noise is shown to share many properties with the AE data. The AE time series is also found to be self‐affine such that the scaling exponent changes at the time scale of about 2 hours. This same time scale is suggested to appear as a spectral break at about 1/(5 hours) in the power spectrum of the AE data.

Planet/magnetosphere/satellite couplings: Observations from the moon

The general characteristics of planetary magnetospheres depend upon a few key parameters, such as the magnetic dipole strength, the planetary rotation rate, and the strength of the internal plasma sources (satellites, rings, ionosphere). The present knowledge of the acceleration and of the large scale circulation of plasma in these magnetospheres is still rather poor. Plasma and energitic particle losses occur largely through precipitation into the atmosphere along magnetic field lines, giving rise to the planetary aurorae. These losses can be initiated by various kinds of magnetospheric processes, and, if clearly understood, could give major insights into the physics of the global magnetospheric system. After a brief comparative review of the planetary magnetospheres, it will be shown how our understanding of their dynamics could benefit from increased instrumental performances in terms of remote sensing in the X rays, UV to IR, and radio wavelength range, and what breakthroughs could be expected from lunar based observations.

Factors influencing the intensity of magnetospheric substorms

A magnetospheric substorm is the sequence of processes which occur throughout the magnetosphere following a southward turning of the interplanetary magnetic field and the onset of dayside reconnection. An isolated substorm consists of three phases, growth, expansion and recovery. Two primary processes make up a substorm. The directly driven component visible in the global, two cell current system is powered by the direct projection of the solar wind electric field onto the conducting ionosphere. The unloading component seen as the one cell intensification of the westward electrojet near midnight is powered by processes internal to the plasma sheet. The intensity of substorms can be measured by a variety of activity indices, the most common of which are magnetic indices. The solar wind controls the intensity of substorms through the process of magnetic reconnection. Time series analysis has established that 40 min after a southward turning of the interplanetary magnetic field the strength of the electrojets is correlated with solar wind parameters through a nonlinear coupling function similar to V 2B Sin 4( θ 2 ) ( where θ = IMF clock angle about Sun vector). Linear filter analysis of the magnetospheric system response shows that the average impulse response for the AL index is a Rayleigh function peaking at a lag time of 60 min and diminishing to zero after 3 h. The average response function for a sequence of moderate substorms is bimodal with peaks at 25 and 70 min, while more intense activity is uni-modal with only the 25 min peak. Response functions for individual substorms have peak amplitudes and delays that can be substantially different from the average. The linear analysis predicts less than half the variance in the AL index when high resolution indices are used. These results imply that reconnection on the day and night side are delayed relative to the solar wind by differing amounts which vary with conditions in the solar wind, magnetosphere, and ionosphere. Currents are driven through the ionosphere that are proportional to the reconnection rates which are in turn proportional to the solar wind electric field, but their strength depends on ionospheric conductivity. Conductivity depends on season, prior activity, and the way in which a particular substorm develops. Computer simulation of substorms with a simple ‘leaky tap’ model suggests that one reason for lack of short term predictabilty is likely to be that the magnetosphere is nonlinear with behavior depending both on previous conditions and the strength and wave form of the solar wind input. New techniques of analysis of magnetic indices support this conjecture. As with the Earth's surface weather, accurate space weather predictions will probably require extensive real-time observatory networks both in the solar wind and magnetosphere.

The nitrogen tori of Titan and Triton

As a result of electron impact dissociation and electron dissociative recombination, nitrogen atoms could be ejected from the exosphere of Titan representing an efficient way of non-thermal atmospheric escape. In this study, the corresponding formation of a nitrogen torus in the magnetospheric system of Saturn is simulated. The number density of the nitrogen atoms in a narrow gas ring near the orbit of Titan is estimated to be on the order of 3–20 atoms cm −3 while a much more diffuse nitrogen cloud covers the whole Saturnian system. Because of the small kinetic energy of the atomic nitrogen fragments at production, only a very small density can be maintained in most parts of the magnetosphere. The ionization of the nitrogen atoms in this extended gas torus could nevertheless introduce an appreciable amount of N + ions into the Saturnian magnetosphere. A similar extended structure of an atomic nitrogen cloud is produced in the Neptunian system as a result of the nonthermal escape of nitrogen atoms from Triton. The peak density near Triton's orbit is estimated to be 10–20 atoms cm −3.

Three-dimensional magnetohydrodynamic simulations of processes at the earth's magnetopause

Abstract Numerical simulation plays a key role in modeling and understanding highly nonlinear two- and three-dimensional plasma processes. A typical process of this type is represented by magnetic reconnection At the Earth's magnetopause magnetic reconnection is believed to be the cause of magnetic flux transfer events or structures which are interpreted in terms of rotational discontinuities. For both observations a clear correlation to a southward interplanetary magnetic field (i.e. large magnetic shear across the magnetopause) is a typical property. From theory and computer simulation it is known that a large magnetic shear is also a requirement for magnetic reconnection. In terms of MHD simulations two major approaches may be distinguished. One of these approaches considers the interaction of the total (dayside) magnetospheric system with the solar wind plasma. These simulations may provide qualitative insight into how the three-dimensional structure of the dayside magnetosphere controls dynamical pro...

Magnetosphere of the rotation-powered pulsar - A DC circuit model

The global structure of the magnetosphere of rotation-powered pulsars is studied by analyzing a large-scale magnetospheric curent system. We assume that this current system forms a DC circuit which includes the electromotive source of the rotating neutron star and three loads : the outer gap, the pair-plasma wind, and the inner gap. First we construct local models for the three parts of the magnetosphere (the two gaps and the wind) with adjustable free parameters, and next we link them to a global model by imposing global link conditions.

Origins of Magnetospheric Plasma

A review is given of recent (1987–1990) progress in understanding of the origins of plasmas in the Earth's magnetosphere. In counterpoint to the early supposition that geomagnetic phenomena are produced by energetic plasmas of solar origin, 1987 saw the pUblication of a provocative argument that accelerated ionospheric plasma could supply all magnetospheric auroral and ring current particles. Significant new developments of existing data sets, as well as the establishment of entirely new data sets, have improved our ability to identify plasma source regions and to track plasma through the magnetospheric system of boundary layers and reservoirs. New computing capabilities have likewise improved our ability to plausibly model the magnetosphere so that new interpretative hypotheses may be tested. These developments suggest that the boundary between ionospheric and solar plasmas, once taken to lie at the plasmapause, actually lies much nearer to the magnetopause. Defining this boundary as the surface where solar wind and ionosphere contribute equally to the plasma, it is referred to herein as the “geopause.” It is now well established that the an infusion of ionospheric O+ plays a major role in the storm‐time distention of the magnetotail and inflation of the inner magnetosphere. This role is often dominant in the larger storms. Smaller quantities of particles with high charge states and very energetic ionospheric molecules are observed as well. However, after more than two decades of observation and debate, the question remains: Are magnetospheric protons of solar or terrestrial origin?

The response of the thermosphere and ionosphere to magnetospheric forcing

During the past six years, rapid advances in three observational techniques (groundbased radars, optical interferometers and satellite-borne instruments) have provided a means of observing a wide range of spectacular interactions between the coupled magnetosphere, ionosphere and thermosphere system. Perhaps the most fundamental gain has come from the combined data-sets from the NASA Dynamics Explorer (DE) Satellites. These have unambiguously described the global nature of thermospheric flows, and their response to magnetospheric forcing. TheDEspacecraft have also described, at the same time, the magnetospheric particle precipitation and convective electric fields which force the polar thermosphere and ionosphere. The response of the thermosphere to magnetospheric forcing is far more complex than merely the rare excitation of 1 km s-1wind speeds and strong heating; the heating causes large-scale convection and advection within the thermosphere. These large winds grossly change the compositional structure of the upper thermosphere at high and middle latitudes during major geomagnetic disturbances. Some of the major seasonal and geomagnetic storm-related anomalies of the ionosphere are directly attributable to the gross windinduced changes of thermospheric composition; the mid-latitude ionospheric storm ‘negative phase’, however, is yet to be fully understood. The combination of very strong polar wind velocities and rapid plasma convection forced by magnetospheric electric fields strongly and rapidly modify F-region plasma distributions generated by the combination of local solar and auroral ionization sources. Until recently, however, it has been difficult to interpret the observed complex spatial and timedependent structures and motions of the thermosphere and ionosphere because of their strong and nonlinear coupling. It has recently been possible to complete a numerical and computational merging of the University College London (UCL) global thermospheric model and the Sheffield University ionospheric model. This has produced a self-consistent coupled thermospheric-ionospheric model, which has become a valuable diagnostic tool for examining thermospheric-ionospheric interactions in the polar regions. In particular, it is possible to examine the effects of induced winds, ion transport, and the seasonal and diurnal U.T. variations of solar heating and photoionization within the polar regions. Polar and high-latitude plasma density structure at F-region altitudes can be seen to be strongly controlled by U.T., and by season, even for constant solar and geomagnetic activity. In the winter, the F-region polar plasma density is generally dominated by the effects of transport of plasma from the dayside (sunlit cusp). In the summer polar region, however, an increase in the proportion of molecular to atomic species, created by the global seasonal circulation and augmented by the geomagnetic forcing, controls the plasma composition and generally depresses plasma densities at all U.Ts. A number of these complex effects can be seen in data obtained from ground-based radars, Fabry-Perot interferometers and in the combinedDEdata-sets. Several of these observations will be used, in combination with simulations using the UCL-Sheffield coupled model, to illustrate the major features of large-scale thermosphere-ionosphere interactions in response to geomagnetic forcing.

Satellite observations of currents and waves in space plasmas

When viewed from outer space, the earth's magnetic field does not resemble a simple dipole, but is severely distorted into a comet-shaped configuration by the continuous flow of solar wind plasma. A complicated system of currents flows within this distorted magnetic field configuration called the ‘magnetosphere’ (See figure 1). For example, the compression of the geomagnetic field by the solar wind on the dayside of the earth is associated with a large-scale current flowing across the geomagnetic field lines, called the ‘Chapman-Ferraro’ or magnetopause current. The magnetospheric system includes large-scale currents that flow in the ‘tail’, the ring current that flows at high altitudes around the equator of the earth, field-aligned ‘Birkeland’ currents that flow along geomagnetic field lines into and away from the two auroral regions, and a complex system of currents that flows completely within the layers of the ionosphere, the earth's ionized atmosphere. The intensities of these various currents reach millions of amperes and are closely related to solar activity. The geomagnetic field lines can also oscillate, like giant vibrating strings, at specified resonant frequencies. The effects of these vibrations, sometimes described as ‘standing Alfvén waves’, have been observed on the ground in magnetic field recordings dating back to the beginning of the century. Observations of currents and waves with satellite-borne magnetic field experiments have provided a new perspective on the complicated plasma processes that occur in the magnetosphere. Some of the new observations are described here.

On the utilization of ionosonde data to analyze the latitudinal penetration of ionospheric storm effects

Upper atmosphere science is placing increased emphasis on global coupling between the magnetosphere, ionosphere, and thermosphere systems, particularly with regard to the penetration of dynamic, chemical, and electrodynamic effects from high to low latitudes during magnetically disturbed periods. An emerging potential exists for latitudinal and longitudinal chains of ionosondes to contribute uniquely to this thrust in ways complementary to the capabilities and shortcomings of other ground‐based sensors and satellites. Here we illustrate a methodology whereby the fullest potential of such ionosonde data can be realized. Data from a chain of stations close to the −165° magnetic meridian and separated by about 5° in magnetic latitude are used to study the relationships between magnetic activity, hmF2, foF2, and inferred meridional winds during 17‐28 April, 1979. Hourly values are fit in latitude using Legendre polynomials, and variations from quiet‐time values are displayed in latitude ‐ U.T. coordinates using a color graphics method which provides an illuminating illustration of the penetration of ionospheric disturbances in latitude and their dependence on Kp, storm time, and local time. Observed effects are interpreted in terms of plausible electric field, neutral wind, and neutral composition changes during the storm period. For instance, net depletions in foF2 occur over the entire disturbed interval down to about 25°‐30° latitude, apparently due to such increased N2 densities that the resulting enhanced plasma loss rates overcompensate any “positive” storm effects whereby southward winds elevate the F‐layer peak to altitudes of reduced chemical loss. Positive storm effects are, however, observed at lower latitudes, and may also reflect the influence of electric fields as well as winds. Interestingly, besides reflecting the anticipated southward flows and equatorward extensions in conjunction with magnetically disturbed conditions, the 24‐hour average meridional winds exhibit a northward return flow after the magnetic disturbance has relaxed; this is a new feature of thermosphere dynamics which should be further investigated using general circulation models and other means of inferring the wind field.

Voyager 2 plasma ion observations in the magnetosphere of Uranus

Positive ion measurements in the magnetosphere of Uranus have been made by the Voyager 2 plasma science experiment. We present an overview of the entire data set and a detailed analysis of the observations from the inner magnetosphere which complements and extends results reported elsewhere. Densities and temperatures are obtained from an analysis which incorporates details of the instrumental response. These results are then used to calculate flux tube particle and energy content to support the hypothesis that the plasma transport is controlled by a solar wind‐driven magnetospheric convection system. Variations in the flux tube content suggest both a local source of plasma, produced from the neutral hydrogen corona of Uranus, and a nonlocal source, convected inward and heated by adiabatic compression. In each case a proton composition is inferred. Sharp boundaries in the high‐energy (approximately 1 keV) plasma population are interpreted in terms of the spatial extent of the magnetospheric convection, with significant shielding of the convection electric field. The convection theory is also used in a simulation of the low‐energy (approximately 10 eV) ion component using the neutral hydrogen source, resulting in distribution functions which qualitatively agree with the observations.

Magnetospheric charge-exchange effect on the electroglow of Uranus

One common feature of the magnetospheric systems of the outer planets explored by the Pioneer 10 and 11 and the Voyager 1 and 2 spacecraft is that extended distributions of neutral gas of planetary exospheric origin and/or of satellites–rings origin coexist with the magnetospheric plasma. This property invariably leads to the occurrence of charge-exchange loss of the charged particles in the inner magnetospheres of Jupiter, Saturn and Uranus. With emphasis on the Uranian system I examine here the idea that the injection of the recombined fast neutrals into the planetary upper atmosphere might be contributing in part to the generation of the dayside electroglow. The recent results from the Voyager 2 encounter also permit the idea that the associated impact ionization effect could be important in maintaining the nightside ionosphere.

Corotating convection revisited

The corotating convection model invokes a systematic large‐scale pattern of plasma motion through the Jovian magnetosphere, a pattern fixed in corotating (System III) coordinates and established by an observed longitudinal asymmetry of the S+ ion distribution in the inner Io torus. In its various versions published to date, the corotating convection model would preserve this longitudinal asymmetry throughout the magnetospheric convection system; the model is thus in conflict with the high degree of longitudinal symmetry that is observed to exist in the hot outer Io torus and presumed to exist also in the magnetosphere outside the torus. We propose to solve this difficulty by superimposing on the corotating convection system a strong retrograde flow confined to the torus region, representing the corotation lag associated with ion production and charge exchange in the Io torus. The addition of this azimuthal flow in the torus region renders the plasma density distribution almost symmetric with respect to longitude in the outer torus and beyond (as required by direct observation), while retaining a corotating convection pattern outside the torus (whose existence is supported indirectly by a number of observations). Serendipitously, the condition of approximate azimuthal symmetry of the plasma density distribution permits an analytic solution of the otherwise intractable equations governing the corotating convection pattern.

What is SPAN?

SPAN is the Space Physics Analysis Network. This important research tool of the NASA scientific community links space researchers from over 50 institutions throughout the United States. The SPAN system is growing within the United States, and it also is expanding to connect NASA scientists with European and Japanese space research institutions.The SPAN system serves many functions. Its paramount purpose is to provide scientists with a tool that improves their productivity. SPAN has traditionally been used to exchange mail messages, to send data back and forth for scientific papers and analysis workshops, and to share scientific software. SPAN has played a crucial role by disseminating spacecraft data in near‐real time during several recent NASA and ESA program successes, such as the International Cometary Explorer (ICE) spacecraft encounter with Comet Giacobini‐Zinner (see “Behind the Scenes During a Comet Encounter” by J.L. Green and J.H. King, Eos, March 4, 1986, p. 105), the Voyager 2 encounter with Uranus, and the Giotto spacecraft encounter with Comet Halley (see “Networking Ground‐Based Images of Comet Halley during the Giotto Encounter” by D. Rees et al., Eos, December 16, 1986, p. 1385). Of course, SPAN has served a variety of broader purposes. It has provided (and will continue to provide) an excellent testing ground for trying new technologies and for evaluating ideas about processing, storing, and transferring various kinds of information. Until the recent availability of computers sponsored by the National Science Foundation, the SPAN network provided one of the few opportunities for NASA researchers to have ready access to the supercomputers needed for large‐scale numerical simulation of magnetospheric and ionospheric plasma systems. SPAN has also increased its usefulness substantially by the addition of the National Space Science Data Center (NSSDC) as a data node on the system. The network, which was originally based on a starshaped configuration, has been redesigned over the last year to take advantage of NASA's new Program Support Communications Network (PSCN).

Plasma Observations Near Uranus: Initial Results from Voyager 2

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

Solar wind energy transfer regions inside the dayside magnetopause: Accelerated heavy ions as tracers for MHD-processes in the dayside boundary layer

Plasma and magnetic field data from PROGNOZ-7 have revealed that solar wind (magnetosheath) plasma elements may penetrate the dayside magnetopause surface and form high density regions with enhanced cross-field flow in the boundary layer. The injected magnetosheath plasma is observed to have an excess drift velocity as compared to the local boundary layer plasma, comprising both “cold” plasma of terrestrial origin and a hot ring current component. A differential drift between two plasma components can be understood in terms of a momentum transfer process driven by an injected magnetosheath plasma population. The braking action of the injected plasma may be described as a dynamo process where particle kinetic energy is transferred into electromagnetic energy (electric field). The generated electric field will force the local plasma to ε× B -drift , and the dynamo region therefore also constitutes an accelerator region for the local plasma. Whenever energy is dissipated from the energy transfer process (a net current is flowing through a load), there will also be a difference between the induced electric field and the v × B term of the generator plasma. Thus, the local plasma will drift more slowly than the injected generator plasma. We will present observations showing that a relation between the momentum transferred, the injected plasma and the momentum taken up by the local plasma exists. For instance, if the local plasma density is sufficiently high, the differential drift velocity of the injected and local plasma will be small. A large fraction of the excess momentum is then transferred to the local plasma. Conversely, a low local plasma density results in a high velocity difference and a low fraction of local momentum transfer. In our study cases the “cold” plasma component was frequently found to dominate the local magnetospheric plasma density in the boundary layer. Accordingly, this component may have the largest influence on the local momentum transfer process. We will demonstrate that this also seems to be the case. Moreover we show that the accelerated “cold” plasma component may be used as a tracer element reflecting both the momentum and energy transfer and the penetration process in the dayside boundary layer. The high He + percentage of the accelerated “cold” plasma indicates a plasmaspheric origin. Considering the quite high densities of energetic He + found in the boundary layer, the overall low abundance of He + (as compared to e.g. O +) found in the plasma sheet and outer ring current evidently reduces the importance of the dayside boundary layer as a plasma source in the large scale magnetospheric circulation system.

Magnetospheric impulse response for many levels of geomagnetic activity

The temporal relationship between the solar wind and magnetospheric activity has been studied using 34 intervals of high time resolution IMP 8 solar wind data and the corresponding AL auroral activity index. The median values of the AL index for each interval were utilized to rank the intervals according to geomagnetic activity level. The linear prediction filtering technique was then applied to model magnetospheric response as measured by the AL index to the solar wind input function VBs. The linear prediction filtering routine produces a filter of time‐lagged response coefficients which estimates the most general linear relationship between the chosen input and output parameters of the magnetospheric system. It is found that the filters are composed of two response pulses speaking at time lags of 20 and 60 min. The amplitude of the 60‐min pulse is the larger for moderate activity levels, while the 20‐min pulse is the larger for strong activity levels. A possible interpretation is that the 20‐min pulse represents magnetospheric activity driven directly by solar wind coupling and that the 60‐min pulse represents magnetospheric activity driven by the release of energy previously stored in the magnetotail. If this interpretation is correct, the linear filtering results suggest that both the driven and the unloading models of magnetospheric response are inportant facets of a more comprehensive response model.