We propose a model to explain how ion dynamics create an Alfvén wave generator in the equatorial region that can be applied to the stable arc problem. For example, in the earthward drifting magnetotail plasma, phase bunching of O+ ions (and to a much lesser extent of the H+ ions) can be caused by a weak (∼1×10−9 Vm−2) electric field gradient [Rothwell et al., 1994]. This leads to density striations in the GSM frame. O+ density striations in the earthward drifting plasma frame are seen as a tailward propagating source of Alfvén waves where the hydrogen ions provide the polarization current of the wave. A transformation to the GSM frame will yield a static, oblique wave structure similar to that previously treated. The waves propagate from the equatorial region to both ionospheres where they are reflected. The ionospheric boundary condition when combined with a magnetospheric boundary condition allows a solution of the wave amplitudes in terms of the striation structure. The frequency of the Alfvén wave and the associated wavelengths are also determined by the striation driver. We find that the magnitude of the parallel current density at the ionosphere has a spatial resonance when the distance between the ionosphere and the equatorial plane is equal to a quarter wavelength along Bo. In that case, the magnitude of the parallel current density at the ionosphere is of the order of 10 μA m−2 and peaks for striation wavelengths (as mapped to the ionosphere) of 10–40 km, which is comparable to the transverse scale of auroral arcs. The associated Poynting flux incident on the ionosphere is found to be ∼ 2 mWm−2 and represents a net transfer of energy from the magnetosphere to the ionosphere as recently observed by experimenters studying substorm onsets. We find that in the steady state the power extracted from the bulk flow to power the arc is balanced by energy provided by the solar wind through the cross‐tail electric field.

In space weather prediction, the transport of solar wind energy through the magnetosphere is a major aspect. For the transport of energy from the magnetosphere to the ionosphere, magnetic field-aligned (Birkeland) currents are a very important agent. In the present paper, we discuss the role of O+ ions for driving field-aligned currents of spatially alternating polarity that may explain multiple auroral arcs, It is known from earlier work that nonadiabatic motion of O+ ions in the magnetotail plasma can lead to the formation of density striations that are stationary in the GSM frame, As the magnetospheric plasma drifts through these density striations, magnetic field-aligned currents of alternating signs are forced to flow in and out of the oxygen-rich region to maintain quasineutrality. This generates Alfven waves that propagate in the drifting plasma but can form stationary structures in the GSM frame. As the currents close in the ionosphere, the equatorial plasma constitutes a generator from which spatially alternating magnetic field-aligned currents carry energy to the ionospheric load, The wavelength of the density striations, mapped to the ionosphere, is compatible with the spacing of stable auroral arcs, and the power supplied by the equatorial generator region is estimated to be compatible with what is needed to drive auroral arcs, Thus, the consequences of nonadiabatic motion of O+ ions may explain how part of the energy extracted from the solar wind is channeled into multiple auroral arcs.

It is shown, by using a circuit model for the magnetospheric current system, that the substorm breakup can be triggered either by some instability anywhere in the circuit or by a decrease in the generator emf, i.e., a northward turning of the interplanetary magnetic field.

Journal of Geophysical Research: Space PhysicsVolume 103, Issue A9 p. 20527-20527 CorrectionsFree Access Correction to “A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases” by V. M. Mishin et al. V. M. Mishin, V. M. MishinSearch for more papers by this authorL. P. Block, L. P. BlockSearch for more papers by this authorA. D. Bazarzhapov, A. D. BazarzhapovSearch for more papers by this authorT. I. Saifudinova, T. I. SaifudinovaSearch for more papers by this authorS. B. Lunyushkin, S. B. LunyushkinSearch for more papers by this authorD. S. Shirapov, D. S. ShirapovSearch for more papers by this authorJ. Woch, J. WochSearch for more papers by this authorL. Eliasson, L. EliassonSearch for more papers by this authorG. T. Marklund, G. T. MarklundSearch for more papers by this authorL. G. Blomberg, L. G. BlombergSearch for more papers by this authorH. Opgenoorth, H. OpgenoorthSearch for more papers by this author V. M. Mishin, V. M. MishinSearch for more papers by this authorL. P. Block, L. P. BlockSearch for more papers by this authorA. D. Bazarzhapov, A. D. BazarzhapovSearch for more papers by this authorT. I. Saifudinova, T. I. SaifudinovaSearch for more papers by this authorS. B. Lunyushkin, S. B. LunyushkinSearch for more papers by this authorD. S. Shirapov, D. S. ShirapovSearch for more papers by this authorJ. Woch, J. WochSearch for more papers by this authorL. Eliasson, L. EliassonSearch for more papers by this authorG. T. Marklund, G. T. MarklundSearch for more papers by this authorL. G. Blomberg, L. G. BlombergSearch for more papers by this authorH. Opgenoorth, H. OpgenoorthSearch for more papers by this author First published: 01 September 1998 https://doi.org/10.1029/98JA02383AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume103, IssueA91 September 1998Pages 20527-20527 RelatedInformation

One of the CDAW 9C substorms is investigated in this paper using the database reported by Hones et al. and supplemented with magnetogram inversion technique (MIT) 2 data. These latter have provided information about the dynamics of the open tail magnetic flux, current systems in the ionosphere, and the size and dynamics of the current wedge. We have identified the growth, expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919‐0935) UT, i.e., between the growth and expansion phases, indicating the concurrent development of the substorm onset and corresponding instabilities in the innermost current sheet, and small‐scale cross‐tail current disruptions without the open tail reconnection. In addition to signatures of small‐scale dipolarization, an increase of the open tail magnetic flux, and a current system of the type close to DP 2 were observed at (0919‐0935) UT, which is more likely to suggest predominance of the tail‐stretching process than magnetic collapse. This fact was interpreted in terms of a relevant simple model as a signature of the growth of the energy input from the solar wind which ensures the observable disturbance power. Hence the disturbance at (0919‐0935) UT was more likely a driven one than an unloading one. The aforementioned signatures make it possible to identify the interval (0919‐0935) UT as the “phase of multiple onsets” or (equivalently) the “first active phase,” which was previously defined by Mishin [1991, and references therein] as one of the four standard phases of a typical substorm (in addition to the expansion phase). Thus the case study supports the substorm scenario with two active phases and, accordingly, with two different kinds of physics. This case study illustrates also the informativity of MIT 2 data and their ability to effectively complement the database traditionally used in substorm studies.

Journal of Geophysical Research: Space PhysicsVolume 101, Issue A12 p. 27461-27461 Replies Reply [to “Comment on ‘Particle dynamics in a spatially varying electric field’ by P. L. Rothwell, M. B. Silevitch, L. P. Block, and C.-G. Fälthammar”] P. L. Rothwell, P. L. RothwellSearch for more papers by this authorM. B. Silevitch, M. B. SilevitchSearch for more papers by this authorL. P. Block, L. P. BlockSearch for more papers by this authorC.-G. Fälthammar, C.-G. FälthammarSearch for more papers by this author P. L. Rothwell, P. L. RothwellSearch for more papers by this authorM. B. Silevitch, M. B. SilevitchSearch for more papers by this authorL. P. Block, L. P. BlockSearch for more papers by this authorC.-G. Fälthammar, C.-G. FälthammarSearch for more papers by this author First published: 01 December 1996 https://doi.org/10.1029/96JA01607AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Cole, K. D., Effects of crossed and (spatially dependent) electric fields on charged particle motion, Planet. Space Sci., 24, 515–518, 1976. Cole, K. D., Dielectric current in a plasma, Phys. Plasma, 3, 2717–2724, 1996. Cole, K. D., Comment on “ Particle dynamics in a spatially varying electric field “ by P. L. Rothwell, M. B. Silevitch, L. P. Block, and C.-G. Fälthammar,J. Geophys. Res., 101(A12)1996. Rothwell, P. L., M. B. Silevitch, L. P. Block, C.-G. Fälthammar, Particle dynamics in a spatially varying electric field, J. Geophys. Res., 100, 14875–14885, 1995. Volume101, IssueA121 December 1996Pages 27461-27461 ReferencesRelatedInformation

Summary form only given, as follows. When magnetic field lines are sufficiently stretched during the substorm growth phase the earthward E/spl times/B ion drift velocity can become comparable to the gyration velocity. Under these conditions inertial currents can become quite important. Using the Rostoker-Skone magnetic field model and a cross tail electric field the authors find that cold (<=200 eV) O/sup +/ ions injected from the ionosphere into the equatorial plane at high latitudes will drift eastward at radial distances less than -10 R/sub E/, due to inertial effects and westward at distanced closer to the Earth due to magnetic drift. The inertial eastward drift gives rise to a current which in terms of J/spl times/B is consistent with the convective deceleration of the earthward drift velocity due to higher values of B. Similarly momentum balance also requires that the convective acceleration of the westward drift velocity consistent with a tailward inertial current. Therefore, an equatorial current wedge system with an eastward and tailward current naturally arises from the ion dynamics and a simple, yet realistic magnetic field model. More complicated current systems can be modelled by adding a radial electric field structure. For example, a weak electric field gradient leads to an earthward inertial current that may dominate the tailward current just discussed. These current structures will be examined for different cross tail electric field values and magnetic field configurations in order to hopefully shed some light on the substorm onset process. Small scale structures related to the phase effects of the oxygen ions will also be discussed.

For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (E×B)/|B|2 as long as the orbit‐averaged value of E is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.

The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region of the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn‐to‐dusk electric field. In combination with the earthward drift motion due to the dawn‐to‐dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross‐tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1 × 10−9 V/m2 ((1 mV/m)/1000 km) at L = 6‐7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic field lines, and such electrons may be the cause of multiple auroral arcs.

Summary form only given. The equations of motion have been solved for ions moving in a model electric field which corresponds to the nightside equatorial region of the magnetosphere. The model electric field has a constant earthward gradient, representing the equatorial counterpart of the Harang discontinuity in the ionosphere, superimposed on a constant dawn-to-dusk electric field. In combination with earthward drift motion due to the dawn-to-dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O/sup +/ ions than for H/sup +/ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The minimum value of the electric field gradient required for the striation to occur in lower for a weaker magnetic field, an enhanced cross-tail electric field, and colder O/sup +/ ions. The first two conditions apply during the growth phase of a substorm.

The experiment is designed to measure the electric field and density fluctuations with sampling rates up to 40,000 samples/sec. The description includes Langmuir sweeps that can be made to determine the electron density and temperature, the study of nonlinear processes that result in acceleration of plasma, and the analysis of large scale phenomena where all four spacecraft are needed.

Previous statistical investigations have revealed a relationship between storm sudden commencements (ssc) and magnetospheric substorm onsets. Little is known about the physical processes constituting this relationship. We used a comprehensive data set for a detailed case study. The ssc occurred on July 6, 1979, at 1930 UT. The substorm expansion phase started 5 min later. The event was preceded by a loading phase of more than 1.5 hours. The loading phase developed in three steps. During each step the cross‐tail current sheet suddenly expanded earthward and intensified. The third step, at 1930 UT, coincided with the ssc. It was very likely caused by the interaction of ions with magnetohydrodynamic waves generated by the ssc. This step was followed at 1935 UT by the onset of the expansion phase accompanied by PiB magnetic pulsations, impulsive electron precipitation, and energetic ion injection at GEOS 2 orbit. The dipolarization of the geomagnetic field started 9 min after the expansion phase onset together with energetic electron injection and a decrease in the energetic ion flux. Signatures of Birkeland currents at dipolarization, and a pressure anisotropy P∥ions > P⊥ions during the 4 min before dipolarization, indicate field‐aligned processes consistent with partial diversion of the cross‐tail current into the ionosphere. The observations are discussed in the framework of a model in which the ballooning mode instability (BMI) developing in the near‐Earth plasma sheet is regarded as the trigger process for the expansion phase. Quantitative estimates of the instability criteria show that the necessary conditions for the BMI are fulfilled during the whole loading phase. An energetic ion pressure gradient, which is needed to drive the BMI, was observed during the interval 1931 UT (ssc) to 1944 UT (dipolarization). Signatures of the instability itself, in terms of regular ion pressure gradient variations, were recorded during the same time interval. We conclude that the magnetosphere was potentially unstable for the BMI when the ssc occurred and that the interaction of the ssc with the magnetospheric particles expanded the cross‐tail current sheet further earthward, thereby creating plasma conditions in which the BMI could grow. The instability started at 1931 UT, and the BMI in turn triggered the expansion phase at 1935 UT.

We find that ions E × B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the magnetotail (Büchner and Zelenyi, 1990; Chen and Palmadesso, 1986; Brittnacher and Whipple, 1991). An analytic expression for the maximum arc width for which chaotic behavior is present is derived and numerically verified. We find, for example, that a 1.5‐km‐thick arc at Λ = 65° requires a minimum potential drop of 3 kV for transverse ion acceleration and heating to occur. Thicker arcs require higher potential drops for stochasticity to occur. This mechanism could be a source for conic ions.

We have recently developed a model for substorm breakup (Rothwell et al., 1988; Rothwell et al., 1989) which indicates that breakup occurrence is very sensitive to the structural details of the individual arc. Specifically, the field-aligned potential drop along the poleward arc boundary must be consistent with the closure of the associated precipitation current in the magnetosphere. The closure is effected by a polarization current which is dependent on the north-south structure of the field-aligned potential drop as mapped to the equatorial plane. Solutions will be presented with particular emphasis on identifying the key geophysical parameters that determine the arc size and its electrical nature relative to our breakup model. We will present a detailed comparison of our model with experimental rocket data.

We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two‐circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north‐south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (Λ) of 62° to hundreds of kilometers above Λ = 68°. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere‐ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.