Simultaneous observations of electric fields, field-aligned currents, and auroral particles on the S3-3 spacecraft over the earth's polar regions have been used to illustrate how the low-altitude extent of field lines open to interplanetary space may be unambiguously identified. We define the polar cap to be the region of open field lines, which are only those containing plasma flowing anti-sunward and possessing precipitating low-energy electron distributions characteristic of the magnetosheath. Field-aligned current signatures are found near the polar cap boundary, but not always coincident with it. Within this polar cap, gradients in the observed soft particle flux are explained as a result of the density decrease previously observed in the magnetosheath as one progresses anti-sunward along the magnetotail. Presumably depending upon the interplanetary magnetic field orientation, the location of the polar cap is occasionally drastically shifted. At these times, there are high-latitude regions, extending even up to the magnetic pole, which are closed and convecting towards the sun. Since boundaries, identified by either the poleward extent of plasma sheet, particle populations or the 33 keV electron intensity cutoff or isotropy boundary, are well-separated at these times from the region of open field lines, we conclude that there is no physical connection between these boundaries and the edge of the polar cap; but, rather, that they are topologically often found near each other. The location of field-aligned current sheets appears to depend, in a more complex way, on the gradients in the electric field and ionospheric conductivity, as indicated by auroral precipitation.

As an illustration of the operation of macroscopic ion acceleration processes within the earth's magnetosphere, we review processes thought to be associated with the formation of the earth's ringcurrent populations. Arguing that the process of global, quasi-curl-free convection cannot explain particle characteristics observed in the middle (geosynchronous) to outer regions, we conclude that the transport and energization of the seed populations that give rise to the ring-current populations come about in two distinct stages involving distinct processes. Near and outside the geostationary region ≳6 to 7 Re), the energization and transport are always associated with highly impulsive and relatively localized processes driven by inductive electric fields. The subsequent adiabatic earthward transport is driven principally by enhanced, curl-free global convection fields.

[1] We believe that a study of auroral morphology is most useful when it can provide some hint of understanding magnetospheric substorms. We found that auroral morphology can throw some light on the presently most controversial issue, namely the so-called “outside-in” model versus the “inside-out” model. In our paper, we pay attention to specific cases of the presence of fairly bright arcs, which are located poleward of the initially brightening arc (the T = 0 arc) and which have so far received not much attention in terms of substorms. The main point raised by Feldstein et al. [2011], as stated in the first paragraph of comments, is that our results are not new. We disagree with this assessment. Our point is that we are not discussing auroral morphology in general, but are examining specific cases, which can help in understanding magnetospheric processes, although a better understanding of auroral morphology is obviously basic. [2] Paragraphs 2 and 3 describe earlier morphological studies, so that we have no disagreement with their statements. It is true that Akasofu [1968] has been misquoted as Akasofu [1964], but we believe that there is a reason for it. Because of the great complexity of auroral displays, our community prefers to have the simplest possible model at this time (which is Akasofu [1968]). In fact, we are still discussing the T = 0 processes and have not paid much attention to other major features such as the poleward expansion, WTS, torches, omega bands, and others. [3] For our specific purpose, taking advantage of moderately active periods, we examined events in which fairly bright arcs were present poleward of the T = 0 arc, which brighten after T = 0. Such arcs are absent in Figure 1. Without such arcs, it is not possible to discuss our findings. (The reason of the missing bright poleward arcs in Akasofu's 1968 model is that because of the complexity of auroral substorms, we were at that time interested in “isolated substorms,” which occur after a prolonged quiet period, avoiding the after effects of preceding substorms.) Note that Akasofu [1964] included arcs poleward of the T = 0 arcs which brighten after T = 0 without mentioning their implications. The growth phase is defined only in terms of magnetic records, and is not well established in terms of auroral morphology, as emphasized by Akasofu and Snyder [1972]. Regarding the reviewers' comment on the growth phase, we believe that it is very dangerous to identify the growth phase only in terms of the AU/AL index. In fact, we examined all-sky camera records during some of the magnetically determined growth phase in published papers and found that the expansion was already well underway. It is generally believed that the growth phase occurs during southward turning of the IMF Bz component. It is for this reason that we examined the IMF data. As stated in our paper, the equatorward shift occurs after the southward turning, but the timing of it is very random as stated, so that we judged that there is no simple one-to-one relationship between the equatorward shifts and IMF southward turnings. We were hoping that the equatorward shift would occur after a definite time delay and that it may be related to thinning of the current sheet. For these reason, as stated in our paper, we withheld to state that the equatorward shift can be associated with the claimed growth phase feature. As recent as in 2007, Petrukovich et al. [2007, paragraph 2] noted, “However, only a few quantitative experimental facts are known about this process.” It is urgent to establish auroral changes after southward turning and during the growth phase. It is well known that during moderately disturbed periods, it is difficult to correlate individual changes in the AU/AL index with specific auroral activities at one location. Thus, in our Figure 4b, we judge that the rapid equatorward motion of arcs that began at about 05:00 UT occurred prior to the following substorm onset, as shown by Snyder and Akasofu [1972]. The dark sky created by the rapid equatorward half between 07:00 and 08:00 UT (after the expansion) and also between 10:30 and 12:00 UT (after the expansion) in Figure 4b clearly shows that our interpretation is correct. The prominent equatorward shift, creating a large dark sky poleward of the T = 0 arc, is absent in Figure 1 of Feldstein et al. [2011]. They show diffuse glow during the expansion phase, not after the maximum epoch of the expansion phase. [4] We have no special comment for paragraph 5, because it is a preparatory note for the following discussions. [5] We believe that the rapid equatorward shift of the equatorward part of the oval, described by Snyder and Akasofu [1972], brings a special meaning in understanding magetospheric substorms, such as current sheet and plasma sheet thinning prior to T = 0. Thus, we are adding its possible implications, not a repeat as Feldstein et al. [2011] implied. It is our hope that theorists will find its significance. In addition to the results in the first part of our paper, namely, that there is no clear indication of specific activities of the northward arcs, the second result implies that auroral substorms are basically a phenomenon that occurs mostly in the equatorward half of the oval, not triggered by outside processes. Therefore, the results have some new implications, in addition to the results by Snyder and Akasofu [1972], which are simply a description of the observation. Such understandings would not result without the presence of the poleward arcs. During moderately active periods, some of the advancing arcs at the front of the bulge from previous substorm activity remain in the poleward sky, allowing us to study for the purpose stated at the outset. We do not think that those arcs are westward traveling surges as suggested by Feldstein et al. [2011]. MSP data show clearly that they are remains of the previous expanded bulge. This is an advantage of studying MSP data. [6] We agree that Poker Flat has only a limited view. It is for this very reason that we examined cases where the poleward expansion stopped well within the view of Poker Flat. As mentioned in paragraphs 6 and 7, our results are additions to those by Snyder and Akasofu [1972], not repeating or contradicting. [7] We agree with their summary of the results by Snyder and Akasofu [1972]. Since we were using MSP data, we could not determine the time of substorm onset. Thus, we were referring only to the arrival of westward traveling surges in our paper, not the substorm onset time. Obviously, the onset time was a little earlier than the arrival time of the surges. However, we disagree with their statement that the growth phase can be identified by the AU/AL index. See also paragraph 4 and paragraphs 6 and 7. [8] We agree that it was difficult to distinguish various types of equatorward motions by all-sky data. By studying MSP data, there is a possibility to do so. Most of them can be described as ‘drifting’ segments of arcs, as illustrated in Figure 1 of Feldstein et al. [2011]. However, we dealt with what can be described best in terms of ‘shift’ of several arcs tightly together. They are different phenomena. We need agreed terms for various equatorward motions. These features have not gotten much attention of theorists. [9] We agree with their statement in paragraph 15. [10] We do not think that poleward arcs occur during their “creation phase.” MSP data show clearly that they are remains of the frontal arcs that moved poleward during the expansive phase. See also paragraphs 4, 6, 7, and 17. [11] MSP data show that the dark space occurs after the expansion phase reached the maximum epoch, not during the expansion phase, namely after the equatorward half of the oval shifts and that some of the expanded arcs remain in the poleward sky. See also paragraphs 4, 6, 7, and 17. [12] It is difficult to see what they claim in Figure 1. [13] Our paper added some important results to those of Snyder and Akasofu [1972]. There is no contradiction between the results obtained by Snyder and Akasofu [1972], Akasofu et al. [2010], as well as Akasofu [1964]. We agree with their second and third main conclusion statements. [14] As stated by Akasofu [2010], we wholeheartedly agree with Feldstein et al. [2011] in their statement, suggesting it is “high time to modify” all the earlier morphological models. There are many forgotten and missing features that are crucial to understanding magnetospheric substorms. Indeed, the purpose of our paper is only a first step toward this goal, which waits for efforts by the younger generations. However, because of the complexity of auroral substorms, it is not an easy task, as Feldstein et al. [2011] point out. Taking this occasion, we would like to emphasize also that the aurora can provide vital information on magnetospheric substorm processes. For this reason, we have to improve auroral substorm models, and unlike single point measurements by satellites, take advantage of the ability to examine a large portion of the sky, corresponding to a large range in the magnetosphere. However, we caution that before they are accepted, new observations should be confirmed by a number of subsequent observations by both the observer and many others. Anything less will result in great confusion. The results of our paper are based on a study of a large number of cases, although only a few examples are presented. [15] Robert Lysak thanks the reviewer for his assistance in evaluating this paper.

Auroral features at about the onset time of substorms are revisited to emphasize their importance in considering substorm onset processes. This study is based on all‐sky camera photographs and meridian scanning photometer records from a single station. First, in considering substorm onset processes, it is crucial to pay attention to the implication of the fact that an auroral arc located just poleward side of the initially brightening arc becomes active only after, not before, onset. Second, prior to substorm onset, there occurs a rapid equatorward shift, or more like narrowing the width, of the equatorward half of the oval, not the whole oval, without any distinct and immediate changes of the interplanetary magnetic field (IMF) Bz component, except that the shift tends to occur after the southward turning of the IMF. These phenomena suggest that some processes take place spontaneously at a distance of less than 10 RE, perhaps near the boundary between the central plasma sheet and the boundary plasma sheet, after the magnetosphere is primed by the IMF southward turning. Thus, it is concluded that substorm models in the near−Earth initiation category satisfy the observational constraints provided by the present paper. Because of our limited data set in terms of observations at single stations, new ground‐based observations are suggested to confirm our results.

The dawn–dusk locations of reconnection in the near‐earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near‐earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn–dusk (Y) locations of relatively fast (≥400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = −5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn–dusk dimension ∼10 RE. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near‐earth reconnection. Auroral breakup is thus likely to always be accompanied by near‐earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.

We investigated whether one or a few coupling functions can represent best the interaction between the solar wind and the magnetosphere over a wide variety of magnetospheric activity. Ten variables which characterize the state of the magnetosphere were studied. Five indices from ground‐based magnetometers were selected, namely Dst, Kp, AE, AU, and AL, and five from other sources, namely auroral power (Polar UVI), cusp latitude (sin(Λc)), b2i (both DMSP), geosynchronous magnetic inclination angle (GOES), and polar cap size (SuperDARN). These indices were correlated with more than 20 candidate solar wind coupling functions. One function, representing the rate magnetic flux is opened at the magnetopause, correlated best with 9 out of 10 indices of magnetospheric activity. This is dΦMP/dt = v4/3BT2/3sin8/3(θc/2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin8/3(θc/2))(interplanetary field magnitude, BT)(merging line length, ∼(BMP/BT)1/3). The merging line length is based on flux matching between the solar wind and a dipole field and agrees with a superposed IMF on a vacuum dipole. The IMF clock angle dependence matches the merging rate reported (albeit with limited statistics) at high altitude. The nonlinearities of the magnetospheric response to BT and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dΦMP/dt. Our results show that a wide variety of magnetospheric phenomena can be predicted with reasonable accuracy (r > 0.80 in several cases) ab initio, that is without the time history of the target index, by a single function, estimating the dayside merging rate. Across all state variables studied (including AL, which is hard to predict, and polar cap size, which is hard to measure), dΦMP/dt accounts for about 57.2% of the variance, compared to 50.9% for EKL and 48.8% for vBs. All data sets included at least thousands of points over many years, up to two solar cycles, with just two parameter fits, and the correlations are thus robust. The sole index which does not correlate best with dΦMP/dt is Dst, which correlates best (r = 0.87) with p1/2dΦMP/dt. If dΦMP/dt were credited with this success, its average score would be even higher.

It is well known that sharp increases/decreases in the solar wind dynamic pressure can result in sudden compression/decompression of the magnetosphere and subsequent magnetic positive/negative impulses (SI+/SI−) detected on the ground magnetometers. While the large‐scale enhancement of aurora during an SI+ has been well established, the response of aurora to an SI− is still little known. This prompts an interesting question whether the response of the global aurora to an SI− mirrors the response to an SI+. This letter reports results from a study of auroral images, acquired from the ultraviolet imager (UVI) on board the Polar satellite, during 13 SI− events. It is found that, in most cases, the luminosity of the aurora indeed showed a clear decrease almost immediately after the decompression. In some cases, the luminosity decrease exhibits a day‐to‐night fading effect and is consistent with the tailward propagation of the magnetosphere decompression front. Auroral particle observations from DMSP indicate that reduction of CPS electron precipitation is the major cause of the large‐scale auroral dimming. We propose that an induction electric field triggered by the sudden expansion of the magnetosphere at the expansion front along with adiabatic decompression and magnetic reconfiguration are responsible for the observed effect.

We have statistically studied magnetotail variations associated with expansion onsets for storm and nonstorm time substorms, using Geotail data. Here storm time was defined as Sym‐H ≤ −30 nT. It was found that there are no qualitative differences in magnetotail variations between these two types of substorms, although the energy accumulation and the dipolarization tend to be more significant during storm time substorms. The statistical results evidently show that the magnetic reconnection and the dipolarization do occur in the magnetotail around onset for both types of substorms, suggesting that storm and nonstorm time substorms are caused by the same mechanism.

Previous work has established that the linear correlation of the low‐altitude particle cusp latitude with the southward component of the IMF is about 0.70. Several possibly better candidate functions for determining the coupling between the magnetosphere and the solar wind have been advanced, but none have been evaluated in terms of the cusp, which is a site of direct solar wind–magnetosphere interaction. On the basis of 11 years of DMSP satellite particle data from 1984–1994 (with verification from the subsequent 11 years, 1995–2005), we find that the best solar wind–magnetosphere coupling function involves electric field dimensions, such as the half wave rectifier ( vBs ) and the Kan‐Lee electric field ( E KL = vB T sin 2 ( θ c /2), where θ c is the IMF clock angle). Both the half wave rectifier ( r = 0.77) and the Kan‐Lee ( r = 0.78) functions have a linear correlation with cusp latitude which is noticeably better than the B z function used in previous work, and also better than the ɛ parameter ( ɛ = vB 2 sin 4 ( θ c /2)). However, the best correlation is with a function whose clock angle dependence is intermediate between the pure half wave rectifier (which implies no merging for B z > 0) and the Kan‐Lee function. Namely, E WAV = vB T sin 4 ( θ c /2) correlates with cusp latitude at the r = 0.81 level. This latter clock angle dependence has been previously suggested at various times by J. R. Wygant, by S.‐I. Akasofu, and by V. M. Vasyliunas. The improved result holds for both the equatorward and poleward edge of the cusp, and regardless of how the IMF is propagated. Earlier work on cross polar cap potentials and on nightside auroral luminosity also favored the E WAV function, which in combination with our findings suggests a widely applicable result. Dayside merging is thus clearly not purely component driven, as the half wave rectifier formula implies. These results also suggest, albeit less convincingly, that merging shuts down for increasingly northward IMF more rapidly than the Kan‐Lee electric field implies.

The two-cell aurora is characterized by azimuthally elongated regions of enhanced auroral brightness over extended local times in the dawn and dusk sectors. Its association with the convection, particle precipitation, and field-aligned currents under various phases of substorms has not been fully understood. With Polar Ultraviolet Imager auroral images in conjunction with Defense Meteorological Satellite Program (DMSP) F12 spacecraft on the dusk-side branch of the two-cell aurora, we are able to investigate an association of the auroral emissions with the electric fields, field-aligned currents, and energy flux of electrons. Results show that the substorm expansion onset does not significantly change the orientation of the dusk-side branch of the two-cell aurora. Also, the orientation of the magnetic deflection vector produced by the region 1 field-aligned current changed from 73±1° to the DMSP trajectory during the substorm growth phase, to 44±6° to the DMSP trajectory during the substorm expansion phase. With a comparison between the orientation of the dusk-side branch of the two-cell aurora and the orientation of the magnetic deflection vector, it is found that the angular difference between the two orientations is 28±5° during the substorm growth phase, and 13±6° during the substorm expansion phase.