Symmetric Ring Current Research Articles

Morphology of the ring current derived from magnetic field observations

Abstract. Our examination of the 20 years of magnetospheric magnetic field data from ISEE, AMPTE/CCE and Polar missions has allowed us to quantify how the ring current flows and closes in the magnetosphere at a variety of disturbance levels. Using intercalibrated magnetic field data from the three spacecraft, we are able to construct the statistical magnetic field maps and derive 3-dimensional current density by the simple device of taking the curl of the statistically determined magnetic field. The results show that there are two ring currents, an inner one that flows eastward at ~3 RE and a main westward ring current at ~4–7 RE for all levels of geomagnetic disturbances. In general, the in-situ observations show that the ring current varies as the Dst index decreases, as we would expect it to change. An unexpected result is how asymmetric it is in local time. Some current clearly circles the magnetosphere but much of the energetic plasma stays in the night hemisphere. These energetic particles appear not to be able to readily convect into the dayside magnetosphere. During quiet times, the symmetric and partial ring currents are similar in strength (~0.5MA) and the peak of the westward ring current is close to local midnight. It is the partial ring current that exhibits most drastic intensification as the level of disturbances increases. Under the condition of moderate magnetic storms, the total partial ring current reaches ~3MA, whereas the total symmetric ring current is ~1MA. Thus, the partial ring current contributes dominantly to the decrease in the Dst index. As the ring current strengthens the peak of the partial ring current shifts duskward to the pre-midnight sector. The partial ring current is closed by a meridional current system through the ionosphere, mainly the field-aligned current, which maximizes at local times near the dawn and dusk. The closure currents flow in the sense of region-2 field-aligned currents, downward into the ionosphere near the dusk and upward out of the ionosphere near the dawn. Key words. Magnetospheric physics (current systems; storms and substorms; magnetospheric configuration and dynamics)

Formation of the storm-time ring current

An extensive study of ring current injection and intensification of the storm-time symmetric ring current is conducted with three-dimensional (3-D) test particle trajectory calculations (TPTCs) in this paper. TPTCs reveal more accurately the process of ring current injection. The main results are the following: (1) an intense convection electric field can effectively energize and inject plasma sheet particles into ring current region within 1–3 h. (2) Injection ions often follow chaotic trajectories in non-adiabatic regions, which may have implications in storm and ring current physics. (3) The shielding electric field, which arises as a consequence of enhanced convection and co-exists with injection and convection electric field, leads the original open trajectories to change into closed ones, thus may play an important role in the formation of the symmetric ring current.

Storm‐time distortion of the inner magnetosphere: How severe can it get?

First results are presented of an effort to model the storm‐time distortion of the magnetic field in the inner magnetosphere using space magnetometer data. Strong geomagnetic storms are relatively rare events, represented by only a small fraction of the data used in the derivation of existing empirical geomagnetic field models. Hence using those models for the mapping of the storm‐time magnetosphere is at most an extrapolation based on trends, obtained from quiet and moderately disturbed data. To overcome that limitation, a set of data was created, containing only clear‐cut events with Dst ≤ −65 nT, with the goal to derive models of the inner and near geomagnetic field (R < 15 RE), representing strongly disturbed geomagnetic configurations and their evolution during the storm cycle. The final data set included about 143,000 records with 5‐min average B‐vectors, covering 37 major storms between 1996 and 2000. Most of the data came from GOES‐8, ‐9, ‐10, Polar, and Geotail spacecraft, and two storms in February–March of 1998 were also partially covered by the data of Equator‐S. In all cases, only those storms were selected for which concurrent solar wind and IMF data were available for the entire duration of the event. Interplanetary medium data were provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail. The inner magnetospheric field was represented using the newly developed T01 model [Tsyganenko, 2002a, 2002b], with a duskside partial ring current with variable amplitude and scale size, an essential part of the storm‐time current system. The modeling revealed an enormous distortion and dawn‐dusk asymmetry of the inner magnetosphere during the peak of the storm main phase, caused by the combined effect of the symmetric and partial ring currents, cross‐tail current, and Birkeland currents. We found that during storms with Dst < −250 nT the tail‐like deformation of the nightside field penetrates so close to Earth that the quasi‐dipolar approximation breaks down at distances as small as 3–4 RE. This finding yields a quantitative answer to the question of why the auroras expand to unusually low latitudes during extremely strong storms. It also may provide a natural explanation for the observed impulsive injections and energizations of charged particles on the innermost L‐shells. Finally, it questions the validity of using the dipole or quasi‐dipole approximation in numerical simulations of severe storms in the inner magnetosphere.

An Approach to the Relationship Between Magnetic Clouds and the Recovery Phase of Geomagnetic Storms

Using an elliptical cross-section model for the study of the magnetic topology of magnetic clouds (MCs) in the interplanetary medium, we develop an analytical approach to their relationship with geomagnetic storms. Assuming an axially symmetric ring current and once we have obtained the disturbances produced in its current density due to the passage of a MC through it (whose axis has a latitude θ, a longitude φ, and its cross-section has an orientation ζ), then we determine the decrease in the value of the geomagnetic field at the Earth's equator, i.e., the Dst index. The Dst model presented allows us to estimate the physical parameters which characterize the symmetric ring current during the recovery phase of the storm time. The theoretical and experimental Dst indexes are compared for four intense geomagnetic storms (Dst<−100 nT), all of them associated with MCs. As seen in the figures presented, the fits are good for every storm. In view of these results we conclude that the effects of a MC over the symmetric ring current can explain the main profile of the recovery phase of a geomagnetic storm.

New Insights on Geomagnetic Storms from Model Simulations Using Multi-Spacecraft Data

The forecast of the terrestrial ring current as a major contributor to the stormtime Dst index and a predictor of geomagnetic storms is of central interest to ‘space weather’ programs. We thus discuss the dynamical coupling of the solar wind to the Earth's magnetosphere during several geomagnetic storms using our ring current-atmosphere interactions model and coordinated space-borne data sets. Our model calculates the temporal and spatial evolution of H+, O+, and He+ ion distribution functions considering time-dependent inflow from the magnetotail, adiabatic drifts, and outflow from the dayside magnetopause. Losses due to charge exchange, Coulomb collisions, and scattering by EMIC waves are included as well. As initial and boundary conditions we use complementary data sets from spacecraft located at key regions in the inner magnetosphere, Polar and the geosynchronous LANL satellites. We present recent model simulations of the stormtime ring current energization due to the enhanced large-scale convection electric field, which show the transition from an asymmetric to a symmetric ring current during the storm and challenge the standard theories of (a) substorm-driven, and (b) symmetric ring current. Near minimum Dst there is a factor of ∼ 10 variation in the intensity of the dominant ring current ion specie with magnetic local time, its energy density reaching maximum in the premidnight to postmidnight region. We find that the O+ content of the ring current increases after interplanetary shocks and reaches largest values near Dst minimum; ∼ 60% of the total ring current energy was carried by O+ during the main phase of the 15 July 2000 storm. The effects of magnetospheric convection and losses due to collisions and wave-particle interactions on the global ring current energy balance are calculated during different storm phases and intercompared.

Storm time equatorial belt – an “image” of RC behavior

During geomagnetic storms a well defined belt of trapped protons and ENAs (energetic neutral atoms) is observed around geomagnetic equator at low L‐values. Their source is RC (ring current) protons existing at larger L‐values. Through charge exchange with the geocorona RC protons become ENAs and if subjected to a new charge exchange become trapped protons. From low latitude particle observations at four different local times we follow; the RC injection region, the drift of RC‐particles through the evening/afternoon into the morning sector, the RC‐asymmetry and convection loss to the dayside during the storm initial and main phase, and its development into a symmetric RC in the recovery phase of the storm.

Variations of Inner Magnetospheric Currents During the Magnetic Storm of May 1998

AbstractTransient variations of the geomagnetic field at low latitudes are mainly caused by the following current systems: the ionospheric dynamo current (IDC), the symmetrical ring current (SRC), and the 3D partial ring current system (PRFI). Meanwhile, the region I ield aligned current‐ionospheric current system (FACI) annot be ignored. In this paper, the ariations of these current systems during the large storm on May 1‐6, 1998 are discussed. The disturbances of the storm contains two events. One is during May 1‐3 and the other is during May 4‐5. The Sq‐field caused by IDC is calculated by using several station pairs. Then, the fields of other sources are separated by fitting data to their field models. Results show that: (1) PRFI and FACI do not develop significantly in the first event, while they develop strongly with substorms in the second event. (2) The variations of SRC‐field are similar to Dst index in the first event and delayed to Dst in the second event. (3) In the second event, PRFI develops before SRC, and SRC depresses associated with the recovery of PRFI and FACI. These results provide a clue to understand the formation of the ring current and field‐aligned currents and their relationship.

Dominant role of the asymmetric ring current in producing the stormtime Dst*

Three storms are examined to determine the contribution to the Dst* index from the symmetric and asymmetric (partial) components of the ring current. The storms (September 24–25, 1998, October 18–19, 1998, and May 14–15, 1997) all have a similar solar wind trigger (an initial shock followed by a coronal mass ejection with southward interplanetary magnetic field) and placement in the solar cycle (rising phase). The near‐Earth ion distribution function is simulated for each storm using a kinetic transport model. The use of a McIlwain magnetospheric electric field description improves the simulation results over the Volland‐Stern field used previously. It is found that most of the main phase magnetic field depression is due to the asymmetric component of the ring current (≥80% at the Dst* minimum for the three storms). Note that this is a minimum asymmetric ring current contribution, because the closed‐trajectory ions may also be spatially asymmetric. Ions in the partial ring current make one pass through the inner magnetosphere on open drift paths that intersect the dayside magnetopause. Changes in the density of the inner plasma sheet are transmitted directly along these open drift paths. For a steady convection field, an increase in the source population produces a decrease (more intense perturbation) in Dst*, while a decrease produces a Dst* recovery. As the storm recovery proceeds, a decrease in the electric field results in a conversion of open to closed drift paths, forming a trapped, symmetric ring current that dominates Dst*. The mostly H+ composition of the ring current for all three storms rules out the possibility of differential charge exchange being the cause of the fast and slow decay timescales, confirming that outflow is the main loss of ring current‐generated Dst* during the early phase decay. The slow decay timescale in the late recovery, however, is dominated by charge exchange with the hydrogen geocorona. The symmetric‐asymmetric ring current is also placed in the context of the solar wind and plasma sheet drivers.

Latitudinal characteristics of the geomagnetic field during the storm of 15–16 July 2000

In this paper geomagnetic disturbances at middle and low latitudes are discussed by using geomagnetic data of the magnetic storm of 15–16 July 2000. This storm is a response to the solar Bastille Day flare on 14 July. Generally, the geomagnetic disturbances at middle and low latitudes during a storm are mainly caused by three magnetospheric–ionospheric current systems, such as the ring current system (RC), the partial ring current and its associated region II field-aligned currents (PR), and the region I field-aligned currents (FA). Our results show that: (1) The northward turning of IMF-Bz started the sudden commencement of the storm, and its southward turning caused the main phase of the storm. (2) The PR- and FA-currents varied violently in the main phase. In general, the field of the FA-current was stronger than that of the PR-current. (3) In the first stage of the recovery phase, the RC-field gradually turned anti-parallel to the geomagnetic axis from a 15° deviation, and the local time (Λ) pointed by the RC-field stayed at 16:00. After that, Λ rotated with the stations, and the RC-field was not anti-parallel to the geomagnetic axis, but 5°–10° deviated. These facts suggest that the warped tailward part of the ring current decays faster than the symmetric ring current.

Comment on “Current understanding of magnetic storms: Storm‐substorm relationships,” by Y. Kamide et al.

With 14 distinguished coauthors and 153 references, this recent paper [Kamide et al., 1998; hereinafter referred to as K98] symbolizes a definitive and authoritative work due a measure of respect. Considered significant enough to be assigned a separate section 7.2, the authors describe reasons that the disturbance storm time (Dst) index, the universal indicator of global magnetic storms, is not the representation of a symmetric ring current (RC) encircling the Earth in the equatorial magnetosphere, as has been supposed by most of the spacescience community since the index design was formalized 34 years ago [Sugiura, 1964]. The K98 authors enumerate a number of factors governing their reasoning: (1) the main contributions of partial ring currents, (2) the existence of strong field-aligned currents, (3) the problem of averaging just four stations for the index, (4) the artificially of making ringsymmetry latitude adjustments, (5) the introduction of false values encountered in removing a “quiet day” field on the nonquiet day of the storm, and (6) the presence of associated magnetotail currents. K98 (p. 17,723) state “Thus the present Dst contains significantly an artificially symmetric value resulting from asymmetric perturbationsz z z” Restated simply, K98 authors say that because of the listed facts it is false to assume that Dst 5 RC. I see two problems with their presentation. First, K98 neglect referencing those who have in the past produced considerable evidence for the major Dst 5 RC inconsistencies that are given in section 7.2. The K98 conclusions are not so obvious that references are unnecessary. K98 coauthors were fully aware of references disputing Dst 5 RC from my presentations of the “Ring-Current Myth”: on October 9, 1992, at the Space Environment Laboratory of NOAA (noted in the work of Campbell, [1996a]); on August 10, 1993 [Campbell, 1993], at the IAGA Assembly in Buenos Aires; and at the February 12, 1996 [Campbell, 1996b], Chapman Conference on Magnetic Storms in Pasadena. In addition to the paper by Campbell [1996a] on the Dst 5 RC myth, I produced an EOS Space Physics and Aeronomy Section News item [Campbell, 1996c] containing such information. K98 authors also could have referred to Campbell [1997, pp. 168–172], the first textbook to caution acceptance of Dst 5 RC in contrast to Mayaud’s [1980] classic rendition of the index. The second problem with the K98 paper is that the authors neglected to present these other important difficulties with the Dst 5 RC assumption: (1) satellite in situ measurements in the RC region fail to find processes paralleling the storm growth and decay phases indicated by the classic interpretation of Dst; (2) determinations within the RC region show insufficient field to account for the surface observations of Dst; (3) there are lunar-tidal effects in Dst that should not occur in Dst; (4) the enhancement of storm time ionospheric currents at the dayside dip equator indicates the existence of strong ionospheric currents providing fields to the low-latitude Dst observatories; (5) the Earth’s midnightside low-latitude fields track the seasonal position and activity changes in the storm time magnetospheric tail-current patterns; (6) the extremely high conductivity of the Earth’s interior shields observatory reception of a partial RC source field from the opposite side of the Earth; and (7) Earthmantle conductivity determinations from an assumed RC source fields are unreliable. It is the ensemble of non-ring-current contributions, not just one or two problems, that destroys the classic interpretation of Dst and begs for another explanation of the characteristic main phase to recovery phase shape of the “magnetic storm.” My publications on the ring-current myth provide accumulated referenced works showing that during a geomagnetic storm period it is wrong to suppose that Dst 5 RC (the symmetric ring current is just one of many contributors to the index) and demonstrating that with Dst showing a unique “storm time” pattern resulting from a summation of many processes and stations, the lognormal distribution characteristics possibly provide a reason for the regularly appearing storm shape (named by Chapman [1951]). The present problem with Dst is to determine how much each process is contributing to the index. Soon, using the equatorial enhancement studies, researchers may be able to extract at least the ionospheric contribution.

Are our ideas about &amp;lt;i&amp;gt;Dst&amp;lt;/i&amp;gt; correct?

Abstract. The idea of two separate storm time ring currents, a symmetric and an asymmetric one has accepted since the 1960s. The existence of a symmetric equatorial ring current was concluded from Dst. However, the asymmetric development of the low-latitude geomagnetic disturbance field during storms lead to the assumption of the real existence of an asymmetric ring current. I think it is time to inquire whether this conception is correct. Thus, I have investigated the development of the low-latitude geomagnetic field during all the magnetic local times under disturbed and quiet conditions. The storm on February 6–9, 1986 and a statistical analysis of many storms has shown that the asymmetry does not vanish during the storm recovery phase. The ratio between the recovery phase asymmetry and the main phase asymmetry is low only for powerful storms. Storms of moderate intensity show the opposite. The global picture of the field evolution of the February storm shows clear differences at different local times. For instance the main phase and recovery phase start time does not coincide with Dst. Also the ring current decay is not the same at different local times. Therefore, Dst gives an incorrect picture of the field development. Moreover, asymmetry does not disappear during international quiet days as the investigation of the low-latitude geomagnetic field shows. Considering all these observations, I think we must revise our ideas about the ring current. In my opinion only one ring current exists and this is an asymmetric one. This asymmetry increases during storms and develops rather fast to more or less symmetric conditions. However, in no case is it justified to conclude from Dst that a symmetric ring current exists.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; storms and substorms)

Ring current and auroral electrojets in connection with interplanetary medium parameters during magnetic storm

Abstract. The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1-3 April 1973. During the storm main phase the DR development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard AE indices calculated on the basis of data from auroral observatories was substantially lower than the real values (AE'). To determine AE' during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (AE') which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. AE' values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between AE' and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) AE index values during a 1-h interval. The ring current magnetic field asymmetry (ASY) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The ASY value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF Bz variations than the symmetric ring current. ASY increases (decreases) faster during the main phase (the recovery phase) than DR. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.

Ring current and auroral electrojets in connection with interplanetary medium parameters during magnetic storm

<strong class="journal-contentHeaderColor">Abstract.</strong> The relationship between the auroral electrojet indices (<i>AE</i>) and the ring current magnetic field (<i>DR</i>) was investigated by observations obtained during the magnetic storm on 1-3 April 1973. During the storm main phase the <i>DR</i> development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard <i>AE</i> indices calculated on the basis of data from auroral observatories was substantially lower than the real values (<i>AE</i>'). To determine <i>AE</i>' during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (<i>AE</i>') which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. <i>AE</i>' values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between <i>AE</i>' and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) <i>AE</i> index values during a 1-h interval. The ring current magnetic field asymmetry (<i>ASY</i>) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The <i>ASY</i> value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF <i>B_z</i> variations than the symmetric ring current. <i>ASY</i> increases (decreases) faster during the main phase (the recovery phase) than <i>DR</i>. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.

Global induction and the spatial structure of mid-latitude geomagnetic variations

SUMMARY A pre-requisite for more precise estimates of the electromagnetic response of the Earth is a better knowledge of the origin and spatial structure of geomagnetic time variations with periods of a few hours to several years. The period range from four hours to four days is particularly crucial for constraining upper mantle conductivity models, but is difficult to analyse because of overlapping contributions from current systems with different spatial scales. A period band centred on two days has been isolated from two years of hourly mean values recorded at up to 130 observatories. Plots of the coherence between pairs of records of the same component of the field at different observatories, as a function of the station separation, prove to be a valuable diagnostic tool. They show that two groups of sources contribute to this part of the spectrum. One group has a spatial scale in excess of 10 000 km, dominates the geomagnetic north (Xm) component, but produces no east (Ym) component. It can be satisfactorily modelled by a P01 spherical harmonic, and must be generated by fluctuations in the symmetric magnetospheric ring current. The spatial coherence of the second group falls to zero in a distance of 5000–7000 km, and can be predicted by a stochastic source model for which the autocovariance scale length is 7000 km. The spatial structure of the two groups is further investigated using the principal component technique of Egbert & Booker (1989). The spectrum is dominated by the largest eigenvalue, and the eigenfunctions corresponding to this mode show the expected P01 source behaviour. The modes which correspond to the next largest eigenvalues are organized into foci approximately 10 000 km across. The vertical fields associated with these modes are of low amplitude at mid-latitudes, but increase steeply beyond 50d. Estimates of the P01 response based upon the horizontal and vertical fields recorded at a single station will be biased by the contribution from the higher order modes. The magnetic observatory network is incapable of describing them properly on a global basis, and only the European subarray is sufficiently dense to form locally valid response estimates.

Mid-latitude auroras and SAR arcs observed from the Isis 2 spacecraft during the August 1972 geomagnetic storm

Optical and particle measurements made from the Isis 2 spacecraft during the August 1972 geomagnetic storm are reported for August 4, 5, and 6. Soon after the two sudden commencements which initiated the storm on August 4 a bright optical aurora was detected in the range of 60°–65° invariant in the dawn sector. On the duskside, only weak aurora was present from 61° to 71°, but there was an 18-kR SAR (stable auroral red) arc at 61° invariant confined to 1° of latitude range and located at the poleward boundary of a broader region of 150-keV isotropic protons. On the morning side, thermally excited 6300-Å emission was apparent only below 55° invariant, much weaker, and much more widespread. As the storm progressed, this mid-latitude emission weakened, but following the major sc at 2054 on August 4 a sharply defined 6300-Å feature was observed at 27° invariant at the inner of two plasmasphere shells observed by Brace et al. (1974). It is concluded that at no time during the storm was a ‘classical’ SAR arc generated, driven by a symmetrical ring current. Instead, an asymmetrical SAR arc generated by inward diffusion of protons during ring current formation was detected, as well as widespread thermal emission arising from unidentified magnetospheric heating processes.

Inductive electric field of a time-dependent ring current

The inductive electric field generated by a time-dependent symmetric ring current has been investigated. The symmetric ring current was modelled by a population of protons drifting in a magnetic dipole field. The interaction of these protons with ion-cyclotron waves was assumed to be the dominant energy loss process for the ring current protons, at least under certain conditions. The calculation, with spectral densities for the ion-cyclotron waves that are based on experimental data, showed that an azimuthal inductive electric field of as much as 0.25 mV/m can be produced by this mechanism. Furthermore there is evidence that if the spectral density of the waves is substantially larger than the one adopted here, the electric field might increase to the order of 1.0 mV/m or more.

Mirror height lowering of trapped electrons by an inductive electric field

The effect of a ring current inductive electric field on the motion of electrons trapped in the magnetosphere is investigated. The lowering of mirror height of the electrons is of primary concern. The guiding center equations are used to describe the particle motion in a dipole field and an inductive electric field that is produced by a time-dependent symmetric ring current. The equations are solved numerically by means of a fourth-order Runge-Kutta technique. Results of the calculation show that a westward inductive electric field of about 0.6 mV/m can lower the mirror heights of electrons to the auroral height (∼100 km) from 200 km in about 290 s. The increase in the collision frequency accompanying such a lowering is 3 orders of magnitude or greater, so that the loss of electrons to the atmosphere should be greatly enhanced.

Energy and momentum theorems in magnetospheric processes

This review deals with the several energy and momentum theorems that relate to magnetospheric processes that have been developed. The region of primary consideration in this paper is the magnetospheric domain that extends between the ionosphere and the interplanetary medium, although, for studying certain phenomena, ionospheric and solar wind properties are of central importance and must be included. Both energy theorems and momentum theorems with their applications are presented. Since energy is an integral property of the system variables, analytical results can be found without knowledge of detailed dynamical processes. Thus, relations are derived between particle and magnetic system energies, and application is made to the shape of the magnetopause and various phases of a magnetic storm. Particular attention is given to symmetric and asymmetric ring currents, including energy and momentum equilibrium conditions; a review of nonlinear self‐consistent models and a discussion of how charge exchange and energy diffusion participate in the recovery phase are presented. Comprehensive expressions for the storm time disturbance field are given in terms of both ring current and boundary current energies, and changes that occur during magnetospheric compressions are discussed. The momentum theorems center around the requirement of static force balance during geomagnetically quiet intervals. Whereas the energy theorems give expressions for the average disturbance field over the earth, the momentum theorems give the gradient in the disturbance field across the earth. The forces between earth and the boundary current, the ring current, and the tail current are derived for various models. It is noted that existing vacuum models of the geomagnetic tail are deficient in meeting the combined requirements of energetics and dynamics of the quiet time tail. Introducing the plasma sheet removes the difficulty by allowing an extra degree of freedom in adjusting the force between the earth and the tail. The role of the plasma sheet in making force adjustments is shown to be consistent with the observed thinning of the plasma sheet before sub storms.

Strong pitch angle diffusion and magnetospheric solar protons

Observations of 1.2- to 2.2- and 2.2- to 8.2-Mev solar protons at low altitudes are studied with emphasis on the interaction zone between the stable trapping regions and the high-latitude polar cap region. The data show: the appearance of anisotropic loss cone distribution, indicative of particle trapping, near the cutoff latitude on the dayside hemisphere only; the motion of the anisotropic region to lower invariant latitudes as the geomagnetic storm develops and the subsequent development of an isotropic loss cone distribution; and the movement of the solar proton distribution to stable trapping regions well within the ≥ 280-kev electron trapping boundary. An explanation for these phenomena is offered in terms of cross-L diffusion in the nightside hemisphere along with the development of some strong pitch angle diffusion process at all longitudes, which drives the ∼1-Mev solar protons to isotropy in the loss cone. The development of strong pitch angle diffusion is strongly linked to the development of the symmetric ring current. This explanation may account for the relative inefficiency of direct solar proton injection as a source of trapped protons in spite of the fact that the solar protons have access to the stable trapping regions. Although the ∼1-Mev proton lifetimes will be governed by the pitch angle diffusion process, it is important to note that the minimum pitch angle diffusion lifetime is of the order of or larger than ∼1-Mev proton drift periods in the region L ≥ 4 RE. These effects may also explain the longitudinal uniformity of PCA on the dayside hemisphere as well as some of the PCA ‘midday recovery.’

The interpretation of surface equatorial magnetic daily variations on disturbed days

Reference is made to the results of an analysis of magnetic data from the lowlatitude station M'Bour—published in the Annls Géophys., 1970 No. 4—from which it is apparent that the contribution of non-ionospheric currents to surface magnetic daily variations is considerable on all days other than magnetically Q days. It is shown that correction for the symmetric ring current using Sugiura's D st values does not completely account for the difference between Q and D days and it is suggested that the discrepancy arises from the asymmetric part of the ring current. On disturbed days at equatorial stations, it is found, from examination of South American IGY data, that in addition to the contributions of the symmetric and asymmetric ring currents, there are superimposed fluctuations in the H component of duration 1–2 hr. These are enhanced around local noon and at the dip equator. In the Z component, there are corresponding fluctuations which appear to be enhanced near the edges of the electrojet region. Possible sources of these fluctuations are suggested.