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Most measures of magnetospheric activity – including auroral power (AP), magnetotail stretching, and ring current intensity – are best predicted by solar wind-magnetosphere coupling functions which approximate the frontside magnetopause merging rate. However radiation belt fluxes are best predicted by a simpler function, namely the solar wind speed, v. Since most theories of how these high energy electrons arise are associated with repeated rapid dipolarizations such as associated with substorms, this apparent discrepancy could be reconciled under the hypothesis that the frequency of substorms tracks v rather than the merging rate – despite the necessity of magnetotail flux loading prior to substorms. Here we investigate this conjecture about v and substorm probability. Specifically, a continuous list of substorm onsets compiled from SuperMAG covering January 1, 1997 through December 31, 2007 are studied. The continuity of SuperMAG data and near continuity of solar wind measurements minimize selection bias. In fact v is a much better predictor of onset probability than is the overall merging rate, with substorm odds rising sharply with v. Some loading by merging is necessary, and frontside merging does increase substorm probability, but nearly as strongly as does v taken alone. Likewise, the effects of dynamic pressure, p, are smaller than simply v taken by itself. Changes in the solar wind matter, albeit modestly. For a given level of v (or Bz), a change in v (or Bz) will increase the odds of a substorm for at least 2 h following the change. A decrease in driving elevates substorm probabilities to a greater extent than does an increase, partially supporting external triggering. Yet current v is the best single predictor of subsequently observing a substorm. These results explain why geomagnetically quiet years and active years are better characterized by low or high v (respectively) than by the distribution of merging estimators. It appears that the flow of energy through the magnetosphere is determined by frontside merging, but the burstiness of energy dissipation depends primarily on v.

Recently it has been shown that isotropic precipitation of energetic protons on the nightside is caused by a non-adiabatic effect, namely pitch-angle scattering of protons in curved magnetic field lines of the tail current sheet. Here we address the origin of isotropic proton precipitation on the dayside. Computations of proton scattering regions in the magnetopheric models T87, T89 and T95 reveal two regions which contribute to the isotropic precipitation. The first is the region of weak magnetic field in the outer cusp which provides the 1–2° wide isotropic precipitation on closed field lines in a ~2–3 hour wide MLT sector centered on noon. A second zone is formed by the scattering on the closed field lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation starting ≈ 1.5–2 h MLT from noon and which joins smoothly the precipitation coming from the tail current sheet. We also analyzed the isotropic proton precipitation using observations of NOAA low altitude polar spacecraft. We find that isotropic precipitation of >30 to >80 keV protons continues around noon forming the continuous oval-shaped region of isotropic precipitation. Part of this region lies on open field lines in the region of cusp-like or mantle precipitation, its equatorward part is observed on closed field lines. Near noon it extends ~1–2° below the sharp boundary of solar electron fluxes (proxy of the open/closed field line boundary) and equatorward of the cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predictions of expected particle scattering in the regions of weak and highly curved magnetic field. We also found some disagreement with model computations. We did not observe the predicted split of the isotropic precipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry line in 10–12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our conclusion is that for both dayside and nightside the isotropic boundary location is basically controlled by the magnetospheric magnetic field, and therefore the isotropic boundaries can be used as a tool to probe the magnetospheric configuration in different external conditions and at different activity levels.

The aurora has been related to magnetometer observations for centuries and to geomagnetic indices for decades. As the number of stations and data processing power increases, just how auroral power (AP) relates to geomagnetic observations becomes a more tractable question. This paper compares Polar ultraviolet imager AP observations during 1997 with a variety of indices. Local time (LT) versions of the SuperMAG auroral electrojet (SME) are introduced and examined, along with the corresponding upper and lower envelopes (SMU and SML). Also, the east-west component, BE, is investigated. We also consider whether using any of the local indices is actually better at predicting local AP than a single global index. Each index is separated into 24 LT indices with a sliding 3 h magnetic local time (MLT) window. The ability to predict AP varies greatly with LT, peaking at 19:00 MLT, where about 75% of the variance (r2) is predicted at 1 min cadence. The aurora is fairly predictable from 17:00 MLT to 04:00 MLT, roughly the region in which substorms occur. AP is poorly predicted from auroral electrojet indices from 05:00 MLT to 15:00 MLT, with the minimum at 10:00–13:00 MLT. In the region of high predictability, the local index which works best is BE (east-west), in contrast to long-standing expectations. However, using global SME is better than any local index. AP is best predicted by combining global SME with a local index: BE from 15:00 to 03:00 MLT and either SMU or SML from 03:00 to 15:00 MLT. In the region of the diffuse aurora, it is better to use a 30 min average than the cotemporaneous 1 min SME value, while from 15:00 to 02:00 MLT, the cotemporaneous 1 min SME works best, suggesting a more direct physical relationship with the auroral circuit. These results suggest a significant role for discrete auroral currents closing locally with Pedersen currents.

OVATION Prime (OP) is an auroral precipitation model parameterized by solar wind driving. Distinguishing features of the model include an optimized solar wind-magnetosphere coupling function (dΦMP/dt) which predicts auroral power significantly better than Kp or other traditional parameters, the separation of aurora into categories (diffuse aurora, monoenergetic, broadband, and ion), the inclusion of seasonal variations, and separate parameter fits for each magnetic latitude (MLAT) × magnetic local time (MLT) bin, thus permitting each type of aurora and each location to have differing responses to season and solar wind input—as indeed they do. We here introduce OVATION Prime-2013, an upgrade to the 2010 version currently widely available. The most notable advantage of OP-2013 is that it uses UV images from the GUVI instrument on the satellite TIMED for high disturbance levels (dΦMP/dt > 1.2 MWb/s which roughly corresponds to Kp = 5+ or 6−). The range of validity is approximately 0

[1] The solar wind impacting the Earth varies over a wide range of time scales, driving a corresponding range of geomagnetic activity. Past work has strongly indicated that the rate of merging on the frontside magnetosphere is the most important predictor for magnetospheric activity, especially over a few hours. However, the magnetosphere exhibits variations on other time scales, including UT, seasonal, and solar cycle variations. Much of this geomagnetic variation cannot be reasonably attributed to changes in the solar wind driving—that is, it is not created by the original Russell-McPherron effect or any generalization thereof. In this paper we examine the solar cycle, seasonal, and diurnal effects based upon the frequency of substorm onsets, using a data set of 53,000 substorm onsets. These were identified through the SuperMAG collaboration and span three decades with continuous coverage. Solar cycle variations include a profound minimum in 2009 (448 substorms) and peak in 2003 (3727). The magnitude of this variation (a factor of 8.3) is not explained through variations in estimators of the frontside merging rate (such as dΦMP/dt), even when the more detailed probability distribution functions are examined. Instead, v, or better, n1/2v2 seems to be implicated in the dramatic difference between active and quiet years, even beyond the role of velocity in modulating merging. Moreover, we find that although most substorms are preceded by flux loading (78.5% are above the mean and 83.8% above median solar wind driving), a high solar wind v is almost as important (68.3% above mean, 74.8% above median). This and other evidence suggest that either v or n1/2v2 (but probably not p) plays a strong secondary role in substorm onset. As for the seasonal and diurnal effects, the elliptical nature of the Earth's orbit, which is closest to the Sun in January, leads to a larger solar wind driving (measured by Bs, vBs, or dΦMP/dt) in November, as is confirmed by 22 years of solar wind observations. However, substorms peak in October and March and have a UT dependence best explained by whether a conducting path established by solar illumination exists in at least one hemisphere in the region where substorm onsets typically occur.

[1] The present study compares isolated and non-isolated substorms in terms of their global morphology and energy deposition. The analysis is based on a list of geomagnetic substorm onsets identified with magnetometer data from SuperMAG and published previously by Newell and Gjerlove (2011a). Isolated substorms are defined as those with separation of two consecutive onsets no less than 3 h. The auroral data are obtained from the global ultraviolet imager (GUVI) on board the TIMED satellite and are rebinned into typical magnetic latitude-magnetic local time maps. The auroral maps are then averaged in 1 min intervals to show the dynamic change of the aurora. The three phases of the substorm are clearly demonstrated in both isolated and non-isolated substorms. However, there are noticeable differences between the two types of substorms: (1) While the nighttime auroral power for both types of substorms slightly increases in the growth phase, isolated (non-isolated) substorms are associated with smaller (greater) nighttime auroral power. (2) In the expansion phase, isolated substorms are associated with greater and more explosive energy release than non-isolated substorms. (3) The time for the recovery phase is ~2 times longer for isolated than for non-isolated substorms. (4) The winter-to-summer auroral power ratio is approximately constant throughout the three substorm phases and the ratio is larger for isolated (~30%) than that for non-isolated (~10%) substorms. It is also found that the polar cap area increases during the growth phase until ~10 min prior to the magnetic substorm onset and decreases rapidly thereafter. The decrease is found to result from the closure of the nightside polar cap associated with substorm expansion. It is found that the observed differences between the two types of substorms simply reflect the differences in the solar wind and EUV drivers. Thus, we conclude that there is no intrinsic difference between isolated and non-isolated substorms in terms of auroral energy release and subsequent auroral power decay.

[1] Substorms release a large amount of energy, some of which is used to energize the precipitating particles in the polar region. Superposed epoch analysis was performed with 11 years of DMSP SSJ/4/5 data to characterize the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions. Although substorms only increase the ion pressure by 30%, they increase the power of the diffuse, monoenergetic, and wave electron aurora by 310%, 71%, and 170%, respectively. Substorms energize the ion aurora mainly in the 21:00–05:00 magnetic local time (MLT) sector. The dynamics of the diffuse electron aurora are different from those of the other two electron aurorae. The expansion phase duration is approximately 15 min for the monoenergetic and wave electron aurorae, whereas it is 1 h for the diffuse electron aurora. The monoenergetic and wave electron aurorae appear to complete the substorm cycle within a 5 h interval, whereas the diffuse electron aurora takes more than 5 h. The diffuse electron aurora power and energy flux start increasing at 15 min before the substorm onset, whereas those for the monoenergetic and wave electron aurorae start increasing at 1 h and 15 min before the onset. The increase in the monoenergetic electron aurora power and energy flux may result from the increase in the magnetotail stretching and region-1 field-aligned current during the growth phase. The monoenergetic electrons may also be associated with fast flows, which have been previously observed more frequently in the dusk-midnight sector.

The suggestion that the polar cap can completely disappear under certain northward IMF conditions is still controversial. We know that the size of the polar cap is strongly controlled by the interplanetary magnetic field (IMF). Under a southward IMF, the polar cap is usually large and filled with weak diffuse polar rain electrons. The polar cap shrinks under a northward IMF. Here we use the global auroral images and coincident particle measurements on May 15, 2005 to show that the discrete arcs (due to precipitation of both electrons and ions) expanded from the dayside oval to the nightside oval and filled the whole polar ionosphere after a long (8 h) and strong (∼5–30 nT) northward IMF Bz, The observations suggested that the polar cap disappeared under a closed magnetosphere.

Polar rain has a beautiful set of symmetry properties, individually established, but not previously discussed collectively, which can be organized by a single unifying principle. The key polar rain properties are favored hemisphere (controlled by the interplanetary magnetic field B x ), dawn/dusk gradient (IMF B y ), merging rate (IMF B z or more generally dΦ MP / dt ), nightside/dayside gradient, and seasonal effect. We argue that all five properties involve variants on a single theme: the further downstream a field line exits the magnetosphere (or less directly points toward the solar wind electron heat flux), the weaker the polar rain. This effect is the result of the requirements of charge quasi-neutrality, and because the ion thermal velocity declines and the tailward ion bulk flow velocity rises moving down tail from the frontside magnetopause. Polar cap arcs (or more properly, high-latitude sun-aligned arcs) are largely complementary to the polar rain, occurring most frequently when the dayside merging rate is low, and thus when polar rain is weak. Sun-aligned arcs are often considered as originating either in the polar rain or the expansion of the plasma sheet into the polar cap. In fact three quite distinct types of sun-aligned high-latitude arcs exist, two common, and one rare. One type of arc occurs as intensifications of the polar rain, and is common, but weak, typically 2 s, and lacks associated ion precipitation. A second category of Sun-aligned arcs with energy flux >0.1 ergs/cm 2 s usually occurs adjacent to the auroral oval, and includes ion precipitation. The plasma regime of these common, and at times intense, arcs is often distinct from the oval which they abut. Convection alone does not specify the open/closed nature of these arcs, because multiple narrow convection reversals are common around such arcs, and the arcs themselves can be embedded within flows that are either sunward or anti-sunward. These observational facts do not neatly fit into either a plasma sheet origin or a polar rain origin (e.g., the necessity to abut the auroral oval, and the presence of ions does not fit the properties of polar rain, which can in any event be nearly absent for northward interplanetary magnetic field). One theory is that such arcs are associated with merging tailward of the cusp. Both of these common types of sun-aligned arcs fade within about 30 min of a southward IMF turning. The third, and rarest, category of sun-aligned arcs are intense, well detached from the auroral oval, contain plasma sheet origin ion precipitation as well as electrons, and persist for hours after a southward turning. These intense detached sun-aligned arcs can rapidly cross the polar cap, sometimes multiple times. Most events discussed in the literature as “theta-aurora” do not fit into this category (for example, although they may appear detached in images, they abut the oval in particle data, and do not have the persistence of detached events under southward IMF turnings). It is possible that no single theory can account for all three types of sun-aligned arcs. Solar energetic particle (SEP) events are at times used to demarcate polar cap open/closed boundaries. Although this works at times, examples exist where this method fails (e.g., very quiet conditions for which SEP reaches below L =4), and the method should be used with caution. Finally, it is shown that, although it is rare, the polar cap can at times completely close.

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 region1 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.

Cusp properties have been investigated with an open-field line particle precipitation model and Defense Meteorological Satellite Program (DMSP) satellite observations. Particular emphasis is placed on the effects of IMF By, since previous studies focus mostly on IMF Bz. The model-data comparisons for various IMF configurations show that the model captures the large-scale features of the particle precipitation very well, not only in the cusp region, but also in other open-field line regions such as the mantle, polar rain, and open-field line low-altitude boundary layer (LLBL). When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower latitude cusp ions originate from the low-latitude magnetosheath whereas the higher latitude ions originate from the high-latitude magnetosheath. The lower latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp. The higher latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz, 30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double cusp result is consistent with the following statistical results: (1) the cusp’s latitudinal width increases with |IMF By| and (2) the cusp’s equatorward boundary moves to lower latitude with increasing |IMF By|.

During the long-duration steady convection activity on 11 December 1998, the development of a few dozen auroral streamers was monitored by Polar UVI instrument in the dark northern nightside ionosphere. On many occasions the DMSP spacecraft crossed the streamer-conjugate regions over the sunlit southern auroral oval, permitting the investigation of the characteristics of ion and electron precipitation, ionospheric convection and field-aligned currents associated with the streamers. We confirm the conjugacy of streamer-associated precipitation, as well as their association with ionospheric plasma streams having a substantial equatorward convection component. The observations display two basic types of streamer-associated precipitation. In its polewardmost half, the streamer-associated (field-aligned) accelerated electron precipitation coincides with the strong (≥2–7μA/m2) upward field-aligned currents on the westward flank of the convection stream, sometimes accompanied by enhanced proton precipitation in the adjacent region. In the equatorward portion of the streamer, the enhanced precipitation includes both electrons and protons, often without indication of field-aligned acceleration. Most of these characteristics are consistent with the model describing the generation of the streamer by the narrow plasma bubbles (bursty bulk flows) which are contained on dipolarized field lines in the plasma sheet, although the mapping is strongly distorted which makes it difficult to quantitatively interprete the ionospheric image. The convective streams in the ionosphere, when well-resolved, had the maximal convection speeds ∼0.5–1km/s, total field-aligned currents of a few tenths of MA, thicknesses of a few hundreds km and a potential drop of a few kV across the stream. However, this might represent only a small part of the associated flux transport in the equatorial plasma sheet.Key words. Ionosphere (electric fiels and currents). Magnetospheric physics (aurroal phenomena; energetic particles, precipitating)

The Polar UV Imager (UVI) is useful both for determining substorm onset times with precision on the order of 1 min and for making quantitative estimates of global power associated with auroral precipitation. Combining these capabilities, we studied 390 substorms, not necessarily isolated, using the superposed epoch analysis technique. The results quantify the phenomenology of auroral power during substorms. Precipitating auroral power rises with three distinct timescales based on distance from the average onset position. The most dramatic results are seen in the location of the auroral bulge, 2100-0000 magnetic local time (MLT). Integrated over this premidnight sector, auroral power decreases by ∼10% in the few minutes before onset. After onset, premidnight auroral power increases by a factor of 3.4 during the first 9 min. The largest increase occurs within 3 min after onset. After peaking at an average of ∼9 GW, premidnight auroral power declines at a steady 0.045 GW min -1 over ∼100 min. Less dramatic increases with longer risetimes are seen elsewhere on the nightside. Outside the LT of the bulge origin, but close enough that the bulge is likely to eventually reach it, the delay from onset to peak auroral power is ∼15-18 min, and the increase is typically in the range of a factor of 2 or 3. Finally, several hours away from the bulge, auroral power increases by less than a factor of 2, with a time delay of ∼36-45 min. Predawn, this represents the travel time of hot eastward drifting electrons in the diffuse aurora. Postdusk (1800-2100 MLT), a fairly weak (between 1.5 and 2.0 fold) increase is seen for unclear reasons. Altogether, a substorm averages 59% greater auroral power dissipation over the entire nightside in the 12() min following onset than in the preceding 120 min (152 and 96 TJ, respectively). By contrast, dayside auroral power is virtually invariant during substorms. Uncertainty in defining onset time using Polar UVI images was less than the 3-min bin period adopted. Auroral onset is thus demonstrated to be well specified.

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large-scale particle precipitation features in the observed cusp, mantle, polar rain, and open-field line low-latitude boundary layer regions. The assumption of a simple dawn-dusk electric field limited the models to the near-noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower-latitude cusp ions originate from low-latitude magnetosheath, whereas the higher-latitude ions originate from the high-latitude magnetosheath. The lower-latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar-orbiting satellite. The higher-latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher-latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz ,30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double-cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with |IMF By| and (2) the cusp equatorward boundary moves to lower latitude with increasing |IMF By|.

Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earth's dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300–0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500–1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed. Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.

Recent research strongly suggests that the ionosphere plays a crucial role in the dynamics of space weather. Although the ionosphere is by volume only a small fraction of the magnetosphere, it serves as a variably conducting boundary, modulating the global electrodynamic circuit in crucial ways. A striking example is the behavior of intense aurora, which have recently been discovered to occur only when the background ionospheric conductivity is low. It is now clear that auroral acceleration occurs at the interface between the ionosphere and the magnetosphere and is controlled by magnetospheric-ionospheric coupling, with the solar cycle variations arising from a surprising source: variations in solar EUV flux. The discovery of diverging electric fields with their possibly corresponding black aurora provides a new symmetry to magnetosphere-ionosphere coupling processes. The far-reaching scope of the ionosphere in space weather problems is illustrated here in several ways. Ionospheric convection is suggested to be a major player in space weather, by creating global coherence in the magnetosphere on timescales not otherwise practical. Even a problem seemingly as far removed as possible from the ionosphere, namely, that of charge neutrality in polar rain (superthermal solar wind electron) entry into the distant magnetotail, is shown to be coupled to the problem of polar wind outflow from the ionosphere.

Global auroral images acquired from the Polar ultraviolet imager in the Northern Hemisphere during the winter of 1996 and the summer of 1997 (4 weeks before and after solstice) are used to study seasonal effects on auroral acceleration and precipitation. The energy flux of precipitating electrons is inferred from auroral luminosity in the long-wavelength bands (1600-1800 A) of N 2 Lyman-Birge-Hopfield (LBH1) auroral emissions, and the mean energy of precipitating electrons is inferred from the intensity ratio of LBH1 to LBHs (1400-1600 A. the shorter wavelength of LBH bands) auroral emissions. Results indicate that dayside and nightside regions of aurora reveal different seasonal effects: nightside (∼1900-0300 MLT) auroral power is suppressed in summer, while dayside auroral power is enhanced in summer and forms the so-called postnoon auroral hot spots, all by a factor of ∼2. The average energy of precipitating electrons is higher in the dark than in the sunlit hemisphere, while the number flux is lower in the dark than in the sunlit hemisphere for all regions. These changes, up to a factor of ∼3, are local time and latitude dependent. The suppression of the nightside auroral power in summer is associated with a large decrease in the electron energy, whereas the enhancement of dayside aurora in summer is associated with a large increase in the electron number flux. The increase of dayside auroral power in summer may be associated with the large-scale upward field-aligned currents, which peak in summer. Results are also discussed in the context of a conductivity feedback instability and a cyclotron maser instability. The asymmetric seasonal effects on the dayside and nightside auroras suggest a voltage generator for the dayside magnetosphere and a current generator for the nightside magnetosphere.

We compare the onset of dispersionless energetic particle injections, observed as a sudden increase of energetic (tens to hundreds of keV) electron and ion fluxes on a timescale of ∼1 min, with the start of auroral breakups. A total of 34 dispersionless injections observed by Los Alamos National Laboratory (LANL) satellites are analyzed, and their corresponding auroral breakups are determined with global auroral images acquired from the Polar ultraviolet imager. An important finding is that dispersionless injections can actually be associated with substorm intensification. The injection time at LANL relative to the start of auroral breakups varies from −2 to 8 min and can sometimes be more than 10 min. The average lag time for the injections compared to the auroral breakups is 1.8 min with a standard deviation of 2.5 min. It is suggested that particle energization must take place in the magnetotail ∼l min earlier than the start of the explosive auroral substorm onset, while the delay of the injections at LANL is due to a propagation effect. An implied average earthward injection boundary is estimated to be ∼ 6.9 – 9.2 RE. Further analysis of the delay time indicates that the transport of substorm injections is associated with the enhancement of convection electric field by a factor of ∼5, corresponding to an earthward convection flow speed of 5 – 120 km s−1. Dispersionless injections can take place in a fairly wide magnetic local time (MLT) region from 2000 to 0100 MLT with a peak at 2200 MLT, where auroral breakups occur most frequently. More importantly, dispersionless injections have ionospheric footprints clustered around the location of auroral breakup within ±1 hour of MLT, further supporting the concept of the close relationship between the substorm injections and the auroral breakups.

A quantitative relationship between auroral brightness and solar EUV produced Pedersen conductance is established by using Lyman-Birge-Hopfleld long and short bands of auroral emissions from Polar Ultraviolet Imager (UVI). We used a conductance model characterized by solar zenith angle, solar F10.7 radio flux, and the local magnetic field to estimate solar EUV Pedersen conductance for each pixel of the images. The hourly average IMP 8 solar wind data set is used to derive corresponding solar wind conditions. Pixels of the auroral brightness in Polar UVI images have been binned in terms of magnetic latitude and local time. We relate the auroral brightness to the conductance and find that a positive correlation occurs in the early morning sector, indicating increasing brightness with increasing conductance. An anticorrelation appears in the premidnight region, supporting the previously reported suppression of the discrete aurora occurrence in sunlight. The correlation becomes stronger for the southward interplanetary magnetic field (IMF) condition than for the all Bz condition in the early morning and premidnight sectors, indicating that the conductance effect on the auroral brightness becomes significant when the IMF is southward. The afternoon auroral bright spot is not eminent when the ionospheric conductance is low.

The Polar Ultraviolet Imager (UVI) instrument can quantitatively determine important magnetospheric descriptors, notably including substorm onset time and global auroral output. Previous research has related the input variables of the magnetospheric system, namely solar wind parameters, to various output variables. However, a complex system such as the magnetosphere includes, in addition to inputs and outputs, state variables. Polar cap flux and magnetotail stretching are two such that can be estimated from the Defense Meteorological Satellite Program (DMSP) series satellites. We herein determine that both polar cap flux, Φc, and the magnetotail stretching index, b2i, do correlate well with 40-min averages of nightside auroral power observed by UVI. There were a total of n = 638 distinct 40-min intervals within which b2i, Φc, and nightside auroral power could be determined. The correlations with premidnight auroral power were r = 0.72 for Φc and r = −0.76 for b2i. The multiple-correlation coefficient of these two variables with nightside auroral power was 0.81. These sample correlations are far better than the sample correlations of solar wind input variables to nightside auroral power. Thus accurate space weather forecasting can demonstrably benefit greatly by monitoring current magnetospheric state variables (nowcasting), rather than attempting to reproduce output variables solely from solar wind inputs. Attempts to predict substorm onsets were less successful. Although the average polar cap flux prior to onset is larger than normal, the difference is not large enough to have significant predictive capability. Specifically, polar cap flux averaged 404 ± 133 and 422 ± 148 MWb for the entire years of 1996 and 1997, respectively, while the polar cap flux at the time of substorm onset averaged 455 ± 143 MWb.

We produce average patterns of global auroral brightness using images from the Polar Ultraviolet Imager (UVI). UVI measures auroral brightness at two wavelength bands within the N2 Lyman-Birge-Hopfield emission spectra, allowing a determination of average energy and total precipitating energy flux. The auroral patterns are organized by both season (to show the effects of solar UV flux on auroral patterns) and interplanetary magnetic field (IMF) orientation. One global pattern for each of eight IMF clock angles in the GSM YZ plane is presented. It is shown that the dependence of global auroral patterns on IMF changes with the season. The afternoon bright spot is more distinct from the nightside aurora for B>y 0 under large northward IMF or small IMF Bz conditions during the summer. The average position of the duskside aurora varies with the sign of IMF By during the summer. However, its location remains almost the same during the winter. Northern Hemisphere auroral power is greater for By 0. The By effect on auroral power is more pronounced when the southward IMF is larger.

We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well-defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67° Λm with half-maximum widths of 3 hours of MLT and 2° Λm, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx > 0 or Bz 0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ∼2200 MLT, whereas in winter they tend to occur near midnight at ∼2300 MLT. The average summer-winter difference in the onset location is ∼1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By > 0 and toward midnight for By 0 in summer and latest (2330 MLT) for By 0 the onset location shifts toward dusk when By > 0 but toward dawn when By < 0; the sense of this shift reverses for Bx < 0. An implication of the results is that auroral breakup is not conjugate.

There are considerable evidence supporting the novel finding from intensive particle observations that electron acceleration events responsible for producing discrete arcs are more common in darkness than in sunlight [Newell et al., 1996a]. This sunlight effect on the production of auroral arcs is most dramatic in the pre-midnight sector, and it is still unclear whether the dayside auroras have the same response to the sunlight as the nightside auroras. In this paper we investigate this issue by using auroral images acquired from the ultraviolet imager (UVI) on board the Polar satellite. By analyzing auroral emission in the long Lyman-Birge-Hopfield band (160 – 180 nm), which is approximately proportional to the total energy flux of precipitating electrons over a wide range of auroral electron energies, one can estimate the total energy deposition rate from auroral precipitation. To emphasize the sunlight effect on auroras seasonal averages of auroral luminosities are derived for the winter of 1996 and the summer of 1997 (4 weeks before and after the solstices). It is shown again that auroral intensity in the premidnight sector is suppressed in sunlight. On the contrary, dayside auroral intensity is generally enhanced in sunlight, indicating a different source mechanism for the dayside aurora from the nightside auroras. The suppression of nightside auroras in sunlight is usually attributed to a feedback mechanism, associated with the ionospheric conductivity, when the nightside magnetosphere acts as a current generator. Since precipitating electrons that produce dayside auroras are associated with upward field-aligned currents, the enhancement of dayside auroras in sunlight can be interpreted as a simple circuit system in which the ionosphere is a load and the dayside magnetosphere is a voltage generator.

On May 11, 1999, when the solar wind almost disappeared, the DMSP-F13 satellite observed two field-aligned current (FAC) sheets on both dawn and dusk sides. The dawnside FACs are identified as conventional region 1 (R1) and region 2 (R2) currents. The duskside FACs are collocated with intense polar-rain precipitation and are identified as midday region 1 and region 0 currents. Whereas the positive IMF BY observed explains the duskward shift/extension of those midday current systems, the extent of the distortion was unusual. It is inferred that the magnetospheric configuration was more axisymmetrical because of extremely low solar-wind dynamic pressure, and therefore the reconnection took place farther away from the midday sector. Conventional R1 and R2 currents as well as boundary layer-type precipitation were nearly absent at dusk, suggesting that the viscous interaction did not operate at least observably on the dusk side in the present event.

A lack of reliable error estimates for poleward auroral emission (PAE) boundaries derived from satellite-borne auroral imagers has hampered the application of these instruments in quantitative magnetospheric energy balance and substorm analysis. In this study, PAE boundaries from Polar Ultraviolet Imager (UVI) images are compared with precipitation boundaries from Defense Meteorological Satellite Program (DMSP) satellite spectrograms. In particular, the study quantifies the accuracy with which UVI images can be used to reproduce the DMSP poleward auroral oval (b5e) boundary. Most of the DMSP b5e boundaries were obtained in the evening sector. It has been determined that a UVI PAE boundary defined by a fixed ratio to the maximum in the auroral oval at each magnetic local time correlates better with the DMSP b5e boundary than one defined by a constant brightness threshold (0.90 versus 0.80 maximum correlation). The optimal threshold and ratio values are found to be 4.3 photons cm−2S−1 and 0.30 normalized flux, respectively. The study also reveals a systematic latitudinal offset between UVI and DMSP in the evening sector with a magnitude of approximately 1°. This offset might represent a real height-dependent geomagnetic influence (e.g., active magnetic topology) or result from systematic errors in the analysis (e.g., removal of the UVI platform wobble). It is demonstrated that the offset can be partially removed with a linear calibration model, allowing the reproduction of DMSP b5e boundaries from UVI images with a standard deviation error of approximately 1°.

A boundary-oriented model of the global configuration of electrons precipitating into the polar ionosphere is presented. It provides the differential energy flux of precipitating electrons from 32 eV to 30 keV for five different activity levels. Data from 12 years and eight DMSP spacecraft were incorporated into the model. The defining characteristic of this model is that only observations similarly located relative to auroral boundaries (e.g., observations just equatorward of the open-closed boundary) are averaged together. The model resulting from this approach more closely resembles instantaneous observations than previous efforts. A distinct polar cap surrounds a narrow auroral zone, transitions between different regions are appropriately sharp, and model spectra are more realistic. This increased fidelity with observation is a significant advantage for the model, broadening its applicability. Also new is the calculation of both mean and median model spectra. The mean is dominated by sporadic flux enhancements, where present, while the median resembles more commonly observed background fluxes, permitting both of these aspects to be addressed. Parameterization for activity is based on the degree of magnetotail stretching, as indicated by the latitude of the ion isotropy boundary. A variety of features can be discerned in the model. There is a large difference between the mean and median energy flux in regions where upward region 1 Birkeland currents are commonly observed. The smooth ∼1 to 10 keV precipitation seen at most local times, in the equatorward portion of the oval, is nearly absent in much of the afternoon sector. Enhanced number fluxes are seen at the poleward edge of the oval near midnight, likely due to the frequent presence of field-aligned bursts. Structured precipitation dominates the energy flux at all local times except between dawn and noon, where the contribution from unstructured precipitation dominates. The total hemispheric energy flux due to mean spectra varies with activity from 6 to 38 GW and exceeds the energy flux due to median spectra by a factor of approximately 4, regardless of activity.

One month's worth of Polar ultraviolet imager (UVI) data were subjected to a “blob” analysis to determine the statistical dynamics of substorm features observed in the Lyman-Birge-Hopfield long (LBHL) band (152–188 nm). Adapted from similar DoD analyses of target images, the analysis consists of finding, on a frame-by-frame basis beginning at substorm onset, the following aspects of an individual auroral feature: peak power (i.e., power of precipitating electrons), total power, centroid location (magnetic local time (MLT) and magnetic latitude (MLAT), and speed of centroid. Over 120 individual auroral features were successfully acquired at onset and tracked until dissipation during January 1997. The power in the peak pixel and total power were random in time but displayed transient spikes that lasted 5–10 min. Over the course of a substorm, the total energy of blobs averaged ∼2.0 − 104 GJ. A histogram of these energies suggests no preferred energy but that lower energies were more common than higher energies. Analysis of the blob positional dynamics generally supports a poleward and westward movement. During the course of a substorm, 90% of the blobs moved poleward, while over 60% moved westward. However, these movements were not steady and displayed random components. Furthermore, a sizable minority (∼35%) of the blobs moved eastward, which does not agree with the conventional picture of auroral surges. Blob speeds varied from essentially zero up to several kilometers per second. However, during the January substorms the blobs did appear to have a preferred speed of 0.84 ± 0.34 km s−1.

Low-altitude satellites make latitudinal cuts through the entire plasma sheet within a few minutes. We show that by combining several such satellites it is possible to form an image of a large portion of the plasma sheet within a relatively short time. Strictly speaking such a technique images the field-aligned portion of the plasma sheet, however, theoretical work as well as extensive in situ observations have demonstrated that the plasma sheet is highly isotropic. In practice the most significant limitation is that electron acceleration events (including the auroral bulge) require discarding the associated ion data. An instance when five DMSP satellites (F10–F14) went through the southern hemisphere nightside oval within a 19 min period is used to construct the first partial magnetotail image. The possibilities of combining data from other missions to construct more complete composite images is considered. This technique is also highly useful in statistical studies of the plasma sheet. Because a low-altitude spacecraft cuts through the plasma sheet about 25 times as often as a mid-altitude spacecraft, and hundreds of times more often than high-altitude spacecraft, statistically meaningful surveys of the plasma sheet as a whole are hundreds of times easier using a collection of DMSP satellites. We demonstrate herein that the dawn LLBL flank is an apparent source of cold magnetosheath plasma supplied to the central plasma sheet.

The shape of auroral zone large-scale electron acceleration events is widely described as an inverted V, with the spectral peak gradually rising to and falling from the spatially central peak. We investigated the relative frequency of all sharp (≥3) electron energy flux enhancement events which are large-scale enough to be resolvable in the DMSP F7 particle data set. The first three days of each month of 1984 were studied, amounting to 355 passes through the nightside oval (restricted to the range 2000–2300 magnetic local time (MLT)) with a total of 1049 such events. Only 42 of these 1049 events actually fit the inverted-V morphology, with another 76 cases fitting an extremely loose definition of an inverted V. Thus, between 4.0 and 11% of all sharp energy flux enhancements are inverted Vs, depending on the strictness of the criteria. In that subset of events for which electrostatic acceleration is likely, namely those which have monoenergetic peaks and which extend over 20 km or more, inverted Vs are still just 10–29% of the total. More common are events which have relatively constant “monoenergetic peaks” (accelerating potentials). A crucial feature common to all non-V forms with monoenergetic peaks is that the maximum accelerating potential (spectral peak) is observed near one edge or both. Thus very large gradients in the potential exist near the edges of large-scale auroral forms, implying much larger electric fields than does an inverted V. The present study thus supports earlier evidence that the important physics of auroral arcs is associated with the edge of the arcs, rather than the central portion of the acceleration region. The most common type of energy flux enhancement event is a broad spectral enhancement (suprathermal burst). Contrary to popular perception, suprathermal bursts are not always narrowly confined in latitude. All categories of sharp electron energy flux enhancement events studied here exhibited a sharp high-energy drop off near the spectral peak, suggesting acceleration of one type or another (either wave-particle or electrostatic).

Birkeland currents transport magnetic tangential stress resulting from J × B forces, which, in the plasma sheet, are balanced by the pressure gradient, Vp. However, derivation of nightside Birkeland currents from Vp observationally has not been possible because pressure must be known everywhere in the plasma sheet at high resolution, which in situ satellites have been unable to provide. Recently, a method of inferring plasma sheet temperature, density, and pressure from low-altitude satellites was developed. The quiet time Birkeland currents (or J // ) are computed from the pressure profile derived from DMSP F8, F9, F10, and F11 data for the year 1992 and magnetic field from a modified Tsyganenko [1989] magnetic field model. Our results show that (1) the region 1 Birkeland currents exhibit a dawn-dusk asymmetry which can be explained by the dawn-dusk asymmetry in the plasma pressure arising if the near-Earth plasma sheet ions are supplied largely by the deep tail plasma sheet and LLBL ions undergoing E X B earthward and gradient/curvature duskward motions; (2) the average region 1 J // is -0.6 and 0.7 nA/m 2 near the neutral sheet (negative current density indicates the currents flow out of the ionosphere and positive means into the ionosphere); and (3) the current system tailward/poleward of the region 1 has the opposite polarities from those in region 1 and apparently is generated from the midnight pressure maximum. These results are fairly consistent with those previously obtained with in situ magnetometer measurements.

The analogy between the dynamic magne- tosphere and an avalanche system is examined by using the global aurora1 energy deposition obtained by the Polar UVI as a measure of the energy output of the magnetosphere. The analysis reveals two types of en- ergy dissipation: those internal to the magnetosphere occurring at all activity levels with no intrinsic scale, and those associated with active times corresponding to global energy dissipation with a characteristic scale. The internal events exhibit the same power law index in both active and quiet times. These features are con- sistent with a simple avalanche ('sandpile') numerical model.

We use observations from five magnetospheric spacecraft in a fortuitous constellation to show that narrow transient plasma flow jets of considerable length formed in the tail can intrude into the inner magnetosphere and provide considerable contribution to the total plasma transport. A specific auroral structure, the auroral streamer, accompanied the development of this narrow plasma jet. These observations support the ‘boiling’ plasma sheet model consisting of localized underpopulated plasma tubes (bubbles) moving Earthward at high speeds as a realistic way to resolve the ‘convection crisis’ and to close the global magnetospheric circulation pattern.

Impulsive Pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes. It is well documented that there is usually more than one Pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Although the Pi2 have become one of the most popular indicators for substorm onsets, the reliability of using the Pi2 in this fashion has not been seriously investigated. In this paper we address this question with a statistical approach by using ∼650 auroral substorm onsets identified with Polar ultraviolet images for a time interval from April 1996 to May 1997. A comparison of the low-latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with the Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as ∼10 min of quiet time preceding the onset) Pi2 bursts seen within ∼10 min from a magnetic positive bay, there were 65 events (∼55%) taking place within 3 min from breakups and 34 events (29%) indicating no sign of an auroral breakup within 10 min of the Pi2 burst. This result suggests that Pi2 may not be as a good indicator of the substorm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When compared to auroral breakups, Pi2 onsets are subject to a small delay of 1 – 3 min, with a peak around l min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast-mode wave, in a plasmapause cavity mode model, propagating in the magnetosphere.

We compare auroral images from the Polar ultraviolet imager (UVI) and simultaneous particle observations from the Defense Meteorological Satellite Program (DMSP) in the afternoon (1300 – 1600 MLT) sector along the oval in the northern hemisphere to determine the magnetospheric source region of postnoon auroral bright spots. Auroral bright spots are determined with Polar UVI, while their magnetospheric source regions are determined from DMSP F13 particle data. A total of 65 events of good temporal and spatial coincidence were identified after searching through over 1 year of data, from April 1996 to June 1997. Instances occur of auroral arcs mapping to each of several different regions, including the plasma sheet, the low-latitude boundary layer, and the plasma mantle. However, our results indicate that ∼2/3 of the time the most prominent auroral arcs are associated with plasma sheet electron precipitation and slightly less than 1/3 of the time they are found to occur near (less than 1° in magnetic latitudes) the boundary between the plasma sheet and other regions.

Precipitating ions on the poleward edge of the nightside auroral oval sometimes exhibit sharp low-energy cutoffs in their energy spectra. These truncated spectra are interpreted as signatures of magnetic reconnection in the magnetotail. The energy cutoff is frequently smoothly dispersed in latitude, allowing an interpretation in terms of quasi-steady reconnection. These events are designated velocity-dispersed ion structures (VDIS) type 2. Roughly one third of type 2 VDIS are accompanied by a sharp transition in the polar rain near the open-closed boundary that aids in their analysis. From 886 nightside open-closed boundary crossings by DMSP spacecraft, 148 type 2 VDIS were identified. They were found most frequently within 2–3 hours of midnight and for 40% of the open-closed boundary crossings between 2200 and 0100 magnetic local time. Minimum variance fits to the cutoff energies and polar rain transition are performed on 49 of these events. For four of them the information from the minimum variance fit and observed convection velocities are used to infer distances to the reconnection site that varied from 30 to 80 RE. In three of these four cases a sharp transition in the convection velocity is observed, coincident with the arrival of ions from the reconnection site. If the reconnection region is viewed as a voltage source, lobe field lines can be insufficiently populated to carry the current necessary to impose the required voltage on the ionosphere. This explains the coincidence between the arrival of ions and a discontinuity in convection, that is, that an electric field from the reconnection site is imposed on the ionosphere but only after sufficient density populates the field lines that connect the regions.

We present the results of a statistical study of auroral pulsations observed at cusp and cleft latitudes as a function of solar wind and geomagnetic conditions. Pulsation periods range from 10 to 40 s but are modulated by longer-period variations ranging from 3 to 5 min. The auroral pulsations are generally accompanied by geomagnetic pulsations, which occur almost every day, whereas the auroral pulsations can only be seen on 20 to 30% of all days studied. The auroral pulsations show a distinct tendency to occur during geomagnetically quiet periods when the interplanetary magnetic field (IMF) has dawnward and northward or only slightly southward components. The auroral pulsations are most common from 0800 to 1100 MLT at 74°–75° geomagnetic latitude. The shorter-period pulsations have latitudinal extents ranging from several 10 s to 100 km, and longitudinal extents of several hundred kilometers. The auroral pulsations show no tendency to occur during intervals of enhanced solar wind velocities or densities, or during intervals of near-radial IMF orientation. Case and statistical studies comparing the latitude at which the pulsations occur with observations by low-altitude DMSP F7 and NOAA 7 spacecraft demonstrate that they occur deep within the magnetosphere on closed magnetic field lines immediately equatorward of the high-latitude trapping boundary for energetic particles. Consequently, we conclude that they do not result from magnetic merging on the dayside magnetopause, the Kelvin-Helmholtz instability, or the transmission of pulsations generated within the Earth's foreshock through the magnetosheath and into the magnetosphere. The origin of the auroral pulsations remains unknown, although it may ultimately be possible to explain them in terms of instabilities at the outer edge of the radiation belt.

During continuous magnetospheric activity it is not uncommon to observe narrow (in MLT) transient particle injections (duration about 1–2 minute at E=100 keV and local time extent ≤ 1 hour MLT) in the nightside part of geosynchronous orbit. Using global UV images from POLAR spacecraft we analyze the development of auroral activity on December 22, 1996 during a sequence of such injections observed by two LANL spacecraft. We found that narrow transient injections are associated with specific localized auroral form, the auroral streamer, which develops in this local time sector. The streamer first appear as a bright spot in the poleward part of the double oval ≈2–5 minutes before the geosynchronous plasma injection, and then develops equatorward, reaching in many cases the equatorward boundary of the UV aurora. We interprete the observations as evidence that some high speed flow bursts (BBFs) of small cross-tail extent (less than 1 h MLT), formed in the distant tail or midtail, can intrude as close to the Earth as the geosynchronous distance before being stopped.

The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval (often associated with the earthward edge of the main plasma sheet), the boundary between smooth and structured precipitation, and the poleward boundary of the particle oval (close to the open-closed field line separatrix). The UV oval is characterized by the latitude of maximum UV intensity, equatorward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundary of the particle oval is often at ≥2° higher latitudes than the most intense UV luminosity. Large differences are typical especially in the midnight and morning sectors. The present results suggest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to compute changes in the amount of open flux under different states of substorm activity.

During the much studied magnetic cloud encounter with Earth on January 10–11, 1997, pairs of polar cap arcs twice became widely separated from the auroral oval, that is, an extremely rare double θ aurora configuration arose. These events exhibit many dynamic features hitherto rarely if ever reported in the literature of θ aurora. The two independent bars proved capable of merging into a single bar, crossing the polar cap, then splitting again. Moreover, in both events, after completely crossing the polar cap, the transpolar arcs reversed direction, and crossed again. The dynamics of these double θ-aurora events appear to constrain any models of their origin. Interestingly, in the January 10 event, a double θ-aurora which had twice crossed the polar cap ended up in a configuration previously termed a double oval. Particle data from the DMSP F13 satellite demonstrates that an isolated plasma sheet fragment formed also in the southern hemisphere, at least for the January 11 event for which correlative data is available.

Auroral breakups are the first visible sign of a substorm expansion onset. Keying the plasma sheet behavior to onset times of auroral breakups may help to identify the substorm onset process. With this goal in mind, we have identified a list of auroral breakups based on global imaging data from the Ultraviolet Imager of the POLAR spacecraft for two favorable viewing periods and have examined the simultaneous plasma measurements in the tail from GEOTAIL. Synoptic patterns of plasma transport and magnetic field changes in the tail surrounding the times of auroral breakups are constructed. The results indicate that the plasma sheet activities associated with auroral breakups are transient and spatially localized. These findings are consistent with the scenario in which expansion phase activities are dominated by localized, transient disturbances as portrayed by the substorm synthesis model.

A flux rope was detected in the near-Earth magnetotail at X ≃ -20 R E by Geotail with simultaneous viewing of the northern polar region by Polar on January 11, 1997. This offers the opportunity to ascertain the optical signature in the ionosphere of a flux rope for the first time. The magnetotail flux rope was associated with tailward streaming of energetic particles and had an estimated core radius of ∼1.5 R E . The composition of energetic ions within the flux rope showed a temporal increase in the abundance of ionospheric N + and O + ions. Simultaneous with the flux rope encounter were auroral activations at -23 MLT (substorm onset) and ∼18 MLT. An extensive consideration of magnetic field projection of the Geotail location to the ionosphere and the associated auroral and particle signatures of this event lead to the inference that the optical signature of the flux rope was an auroral arc system at ∼18 MLT, which had an ∼2 h extent in MLT and was moving progressively poleward and westward.

An extended interval of strong northward interplanetary magnetic field (IMF) was observed by the Wind spacecraft located at an upstream distance of ∼193 RE from February 8–10, 1995, with a brief break of southward IMF from 0200 to 0400 UT on February 9. This brief interval of southward IMF led to an isolated substorm of moderate intensity (∼500 nT) with expansion phase starting at ∼0431 UT. This substorm may be triggered by the northward turning of the IMF since its onset time matched well with the time expected for the arrival of the northward turning of the IMF at Earth. The substorm activities were monitored by 11 spacecraft in space (Wind, IMP 8, Geotail, six geosynchronous satellites, one DMSP satellite, and Freja) and two networks of ground stations (Canopus and SuperDARN) covering both the northern and southern hemispheres. The extensive coverage of this event provides us with results (1) showing some unusual characteristics possibly related to the isolated nature of the substorm and (2) revealing some surprising features difficult to reconcile with the traditional substorm model. In the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit. In the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site (X line) moving from the near-Earth tail to the more distant tail. We also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail.

Using magnetic field or particle data obtained by the Magsat, DMSP-F6, and DMSP-F7 satellites, we have investigated characteristics of the magnetospheric plasma environment and field-aligned currents (FACs) during prolonged geomagnetic quiescence (Kp = 0) associated with extremely low values of the interplanetary magnetic field (|By| ≤ 1.5 nT, −0.5 nT ≤ Bz ≤ 1.5 nT). We refer to this as the “ground state” of the magnetosphere. The large-scale FACs exhibit the traditional region 1/region 2 current pattern and the dayside region 0 (traditional cusp) current system. The median region 1 current intensity at 835-km altitude is 135 nT (with half of the values contained in the range 126–156 nT) in the 0800–1600 magnetic local time (MLT) region excluding the cusp. These are much smaller than the region 1 peak intensities of 740 nT for the |AL| ≥ 100 nT case and 350 nT for the |AL| < 100 nT case as statistically determined by Iijima and Potemra [1978]. In addition, the region 1/region 2 FAC systems tend to degenerate into smaller-scale FACs at local times separated from the dayside toward morning and evening hours. The median region 1 intensity is 70 nT (with one-half the values between 52 and 96 nT) in the 2200–0200 MLT sector. In the prenoon and postnoon sectors, region O currents with intensities comparable to region 1 continue to be paired with region 1 currents. The region 0 and region 1 currents on the dayside extend in the azimuthal direction, are balanced in current intensity with a typical value of 140 nT in the 0915–1000 MLT sector, and are collocated with boundary layer particles. These observations have provided the first opportunity to study the basic flow patterns of field-aligned currents during the magnetospheric ground state.

A method of inferring central plasma sheet (CPS) temperature, density, and pressure from ionospheric observations is developed. The advantage of this method over in situ measurements is that the CPS can be studied in its entirely, rather than only in fragments. As a result, for the first time, comprehensive two-dimensional equatorial maps of CPS pressure, density, and temperature within the isotropic plasma sheet are produced. These particle properties are calculated from data taken by the Special Sensor for Precipitating Particles, version 4 (SSJ4) particle instruments onboard DMSP F8, F9, F10, and F11 satellites during the entire year of 1992. Ion spectra occurring in conjunction with electron acceleration events are specifically excluded. Because of the variability of magnetotail stretching, the mapping to the plasma sheet is done using a modified Tsyganenko [1989] magnetic field model (T89) adjusted to agree with the actual magnetotail stretch at observation time. The latter is inferred with a high degree of accuracy (correlation coefficient -0.9) from the latitude of the DMSP b2i boundary (equivalent to the ion isotropy boundary). The results show that temperature, pressure, and density all exhibit dawn-dusk asymmetries unresolved with previous measurements. The ion temperature peaks near the midnight meridian. This peak, which has been associated with bursty bulk flow events, widens in the Y direction with increased activity. The temperature is higher at dusk than at dawn, and this asymmetry increases with decreasing distance from the Earth. In contrast, the density is higher at dawn than at dusk, and there appears to be a density enhancement in the low-latitude boundary layer regions which increases with decreasing magnetic activity. In the near-Earth regions, the pressure is higher at dusk than at dawn, but this asymmetry weakens with increasing distance from the Earth and may even reverse so that at distances X less than approx. 10 to -12 R(sub E), depending on magnetic activity, the dawn sector has slightly higher pressure. The temperature and density asymmetries in the near-Earth region are consistent with the ion westward gradient/curvature drift as the ions ExB convect earthward. When the solar wind dynamic pressure increases, CPS density and pressure appear to increase, but the temperature remains relatively constant. Comparison with previously published work indicates good agreement between the inferred pressure, temperature, and density and those obtained from in situ data. This new method should provide a continuous mechanism to monitor the pressure, temperature, and density in the magnetotail with unprecedented comprehensiveness.

Recently, efforts to characterize and monitor the state of the magnetosphere have intensified, along with the rising interest in space weather. The latitude of the ion energy flux precipitation maxima (“b2i”), which almost invariably occurs near the equatorward edge of the nightside main auroral oval, has been suggested as one such parameterization. It has been suggested that b2i corresponds to the ion isotropy boundary (IB), which has been independently researched as a measure of the extent to which the magnetotail is stretched. By comparing simultaneous observations by the Defense Meteorological Satellite Program (DMSP) and NOAA spacecraft, we confirm a close association between b2i and the isotropy boundary of 30 keV protons. Using 2.5 years of simultaneous data from DMSP and GOES spacecraft, we verified that magnetic field inclination (the extent to which the magnetotail is stretched) strongly controls the b2i/IB latitude. Based on use of the b2i latitude, corrected for local time variation, as an index of magnetic stretching in the tail to show a considerable dawn-dusk asymmetry, we find that the magnetic field is more depressed and stretched at dusk than at dawn, and asymmetry increases with increasing magnetotail stretching. This asymmetry is consistent with the rotation of the symmetry line of the b2i(MLT) curve toward premidnight hours and suggests the growth of a so-called “partial ring current” system with increasing activity. Finally, the utility of the b2i/IB boundary as a characterization of the state of the magnetosphere is shown by demonstrating that the average pressure in the magnetotail is better specified by b2i than by Kp.

The shape of the open–closed boundary is studied using auroral oval crossings from up to four DMSP satellites, providing a maximum eight-point determination per hemisphere. The spectra of both precipitating ions and electrons are examined, and boundary crossings are determined by visual inspection. A subset of crossings with between six- and eight-point determinations observed during intervals spanning 15 to 58 min are used to form a cubic spline approximation to the open–closed boundary for each interval. The variability in the size, shape, and location of the boundary is characterized. Approximately half the time the points can be well fit by a circle, but for the remaining intervals the shape is more complex. The speed at which the boundary moves is estimated, and the accuracy of determining the amount of open flux from n observed boundary crossings is tested. When only one point is used to determine the open flux (by assuming an offset circle), the standard deviation of the relative error is 33%, dropping to 16% for a four-point measurement. The variation of the open flux with measures of solar wind coupling is tested and found to be roughly proportional. These findings have implications for space weather applications which require the construction of a data set of open flux.

A SuperDARN HF radar pair (Saskatoon-Kapuskasing) has been used to measure Doppler spectra of backscatter from the dayside F-layer. Normally the spectra are single-peaked, but a small fraction exhibit a double-peaked (D-P) signature. On the 2D convection maps, these D-P spectra occur in range cells located poleward of the convection reversal. A comparison with DMSP SSJ/4 particle data measurements and their mapping to magnetospheric boundaries shows that the D-P spectra are concentrated just equatorward of the magnetopause, in regions of spatially/temporally structured soft electron precipitation (about 300 eV) where the highly variable flux can reach 2–5 ergs/cm²/s. The D-P spectra are most easily explained in terms of scattering from small-scale vortices of size less than the radar resolution of 45 km. The D-P spectral measurements are illustrated by SuperDARN and DMSP data for a dayside event on Feb. 20, 1995, when northward IMF conditions prevailed. We conclude that HF radar D-P observations can be used to map in real time the dayside 2D ionospheric footprint of the outer LLBL.