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International audience; We examined some 75 observations from the low-altitude Earth orbiting DMSP, Ørsted and CHAMP satellites which were taken in the region of the nominal cusp. Our objective was to determine whether the actually observed cusp locations as inferred from magnetosheath-like particle precipitation (“particle cusp”) and intense small-scale magnetic field variations (“current cusp”), respectively, were identical and were consistent with the statistically expected latitude of the cusp derived from a huge number of charged particle spectrograms (“statistical cusp”).The geocentric coordinates of the satellites were converted into AACGM coordinates, and the geomagnetic latitude of the cusp boundaries (as indicated by precipitating particles and small-scale field-aligned currents) set in relation to the IMF-Bz dependent latitude of the equatorward boundary of the statistical cusp.We find that the actually observed latitude of the particle cusp matches well the statistically expected latitude while the current cusp appears to cover most of the statistical cusp and also a ≈1° wide section beyond the equatorward boundary of the statistical cusp. This leads us to suggest that intense small-scale field-aligned currents are generated in the cusp but also in the transition zone between the low-latitude boundary layer (LLBL) and the cusp, probably within both regimes, the cusp and the open LLBL. The small-scale field-aligned currents are possibly a consequence of turbulence and/or instabilities associated with the process of opening previously closed magnetospheric field lines and merging them with the interplanetary magnetic field.

Controversy continues unabated concerning the detection of signatures of merging at low altitudes. One phenomenon required by nearly all models of merging is the acceleration of ions in the merging region by about the Alfven velocity. We report the observation of spatially discrete patches of precipitating accelerated ions at the equatorward edge of the cusp. These regions of accelerated bulk flow velocity, which occur at or near the transition between boundary layer and cusp proper, are spatially distinct from the overall energy-latitude dispersion in the main cusp and are best fitted by Maxwellian distributions with convection velocities of 450-640 km/s; such velocities are 200–300 km/s faster than in the main cusp ion distribution. The acceleration cannot be attributed to electrostatic potentials, for the electron distributions in the patchy ion regions are also accelerated. Based on an initial survey of 13 such events, occurrence of distinctly separate accelerated ion patches appears to be primarily or exclusively a southward Bz phenomenon.

Three months of ground-based recordings of high latitude dayside Pc1 magnetic variations at auroral oval stations Syowa (MLAT = 67°) and Mawson (MLAT = 70°) are compared. Dynamic spectra reveal that banded unstructured Pc1 emissions (0.2–0.4 Hz) may be separated into pre-noon and post-noon activity. Although the pre-noon activity is common at Mawson, its observation at Syowa is associated with low altitude cleft-like ions being present at lower than normal latitudes, i.e. 75° MLAT. These ions are determined from DMSP satellite observations. The Pc1 emissions at both stations are interpreted as arising from ion cyclotron waves generated in the equatorial plasmatrough region. We regard the dayside cleft as the free energy source of the pre-noon Pc1 activity by supplying the equatorial magnetosphere with ions. It is believed that the ion cyclotron waves enter the ionosphere within the auroral oval, poleward of Mawson and Syowa and fast Alfven mode waves, which then arise, propagate to the stations in the F2-region ionospheric waveguide.

[1] ACE SWEPAM measurements of solar wind field-aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross-polar-cap gradients in the intensity of otherwise-steady polar rain. The generally good agreement of the distribution functions, f, from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field-aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low-energy electrons and maintain charge quasi-neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field-aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.

[1] Analysis of a sequence of auroral images acquired by the Polar Ultraviolet Imager (UVI) during a long-duration (∼1.5 hr) solar wind dynamic pressure enhancement reveals auroral intensity variations in the Pc5 band. Power spectral analysis indicates that the “auroral pulsations” appeared in multiple frequency bands predominantly in the day sector and the dawn-dusk flank of the oval, with the maximum wave power skewing toward the dawn sector. Ground-based magnetometer recordings showed Pc5 micropulsations modulated with the auroral pulsations. The lower frequency (1–3 mHz) auroral pulsations were modulated with solar wind dynamic pressure pulses moving tailward, whereas the higher frequency (6–8 mHz) ones were not. We concluded that the lower frequency auroral pulsations were driven directly by the solar wind, whereas the higher frequency ones were associated with global compressional cavity mode. This study shows for the first time the snapshot of global Pc5 ultralow-frequency (ULF) waves and demonstrates that global far-ultraviolet (FUV) imaging such as Polar UVI provides a powerful ancillary tool for studying ULF waves on a global scale.

[1] We investigated the behavior of 10 different characterizations of the magnetosphere, including traditional geomagnetic indices such as Kp and AE. We also used satellite based data such as global auroral power from Polar UVI, cusp latitude from DMSP, and magnetotail stretching from GOES. Multiyear data (typically a solar cycle) was studied at relatively high cadence (usually 1 h) to provide better statistical consistency. Simple two parameters best fits to a wide variety of candidate solar wind coupling functions were considered, with no hidden variables or adjustable parameters. Previously we showed that the best performing solar wind coupling functions all proved to be various estimators of the global merging rate, with the best results from dΦMP/dt = v4/3BT2/3sin8/3(θc/2). Here we investigate the best performing viscous candidates, and the best performing pairs of solar wind coupling functions, in predicting these same 10 characterizations of the magnetosphere. The top viscous functions all are closely related to the solar wind pressure, but n1/2v2 performs best, accounting for 22.3% of the variance, versus 14.7% for v and 12.5% for p. Altogether we considered 32 different candidate solar wind magnetosphere coupling functions, and all 496 unique pairs of functions. Because of the large number of function pairs, some statistical fluctuations are expected, and indeed observed, in predicting individual indices. Nonetheless, certain patterns emerged. The best performing overall pair (predicting 61.0% of variance across all indices) was dΦMP/dt coupled with n1/2v2, i.e., the best individual merging and best viscous terms make the best combination. All the top pairs consisted of one merging and one viscous term. Combining two distinct estimators of the merging rate always has less predictive power than combining a viscous and merging term. However, any merging term, such as Bs, vBs, or EKL, when coupled with almost any viscous term, such as v, p, n1/3v2, p1/2, etc, performs reasonably well, with the merging term invariably accounting for much the greater fraction of variance.

The Geospace Environment Modeling (GEM) Program is directed toward modeling the coupled solar wind/magnetosphere/ionosphere system. The inter-calibration of ground-based observations of the ionosphere and satellite observations has been identified as an essential step in tying together the data to produce a global picture of geospace. On October 10, 1988 the DMSP-F9 satellite passed through the Southern Hemisphere cusp while a coheret scatter HF-radar was observing 10-m scale irregularities present in the ionosphere. The combined data indicate that these irregularities were being generated in the cusp, and that the cusp was a region of greater than normal electric field turbulence. The radar data indicate that the cusp was colocated with the region where the ionospheric convection rotated from sunward to anti-sunward with increasing latitude. These observations provide an unambiguous case where simultaneous satellite and ground-based observations of the cusp can be compared.

[1] The transient behavior of shock-induced auroras reported by previous workers suggests a cause-and-effect relationship between the shock front and the auroral transient. However, it is not known if the high solar wind dynamic pressure downstream of a shock can have significant effects on the global auroral morphology. In this brief report we present results from an observational study on global auroral response to large solar wind dynamic pressure, using global auroral images acquired by the ultraviolet imager on board the Polar spacecraft. It is found that the luminosity of the aurora showed a general prompt and lasting increase in the dawn and dusk flanks of the oval. Such an auroral activity is closely related to the compression of the magnetosphere and is thus christened “compression aurora.” The luminosity increase, though predominantly in the dawn sector of the oval, may persist as long as the solar wind dynamic pressure stays high and may disappear ∼10 min after the dynamic pressure drops. A detailed examination of Defense Meteorological Satellite Program particle data on one fortuitous event on 26 September 1999 indicates that the compression aurora is produced mainly by precipitating electrons originating from the central plasma sheet (CPS). Plasma wave observations made from Geotail in the dawn sector of the near-Earth plasma sheet indicates no expected plasma wave activity that may have been otherwise considered a primary cause of enhanced CPS particle precipitation through a pitch angle loss by wave-particle interactions. We propose that reductions of the Earth's magnetic field mirror ratio by magnetospheric compression can be a prime mechanism for the compression aurora as long as there is a continuing supply of particle source.

[1] The traditional distinction between the cusp for northward and southward interplanetary magnetic field (IMF) conditions is deficient in that many, if not most, “northward” IMF cusp encounters closely resemble southward IMF encounters. Partly for that reason, “southward” IMF cusps have been far more often examined than “northward. ” Recently, Newell et al. (2007) showed that the magnetopause merging rate, dΦMP/dt, much better predicts cusp latitude than does Bz (or Bs). Here, we investigate the extent to which high and low merging rate conditions better separates cusp encounters into mutually distinct classes. Indeed, high merging rate cusps (those with dΦMP/dt > 2* 〈dΦMP/dt〉) differ from low merging rate cusps (those with dΦMP/dt < 0.5〈dΦMP/dt〉) in several quantitative and even qualitative ways. High-energy (tens of keV) ions, apparently of magnetospheric origin, do not extend into the high merging rate cusp but frequently do for low merging rates. However, the dispersion curve for ions, apparently of magnetosheath origin, extends to several keV for high merging rates but only to 1–2 keV for low merging rates. The local time extent of the cusp is 2.33 hours MLT for low merging rate conditions and 3.45 hours MLT for high merging rate conditions. All high merging rate cusps show clear forward dispersion (declining energy with increasing latitude) with low-energy ion cutoffs within that dispersion. Low merging rates cusps rarely show forward dispersion, but about half, or slightly less, show reverse dispersion (declining energy with decreasing latitude) at the poleward edge of the cusp. Low-energy ion cutoffs in the low-merging rate cusp primarily occur only within that reverse dispersion. “Double” cusps, some with very clear latitudinal separation, occur in some high merging rate cases, but no low merging rate cases. The high merging rates cases also typically have a “shadow” region of electron-only precipitation at roughly polar rain energies and intensities, lying immediately equatorward of the main cusp. For low-merging rate conditions, the region immediately equatorward of the cusp (main magnetosheath ion population) contains ions as well as electrons, and forms a boundary layer.

Received 23 August 2006; revised 12 February 2007; accepted 1 March 2007; published 23 June 2007. [1] We study the statistical characteristics of four-sheet structures of large scale field-aligned currents (FACs) with an emphasis on particle precipitation collocated with each FAC sheet observed in dayside using magnetic field and particle precipitation data from the Defense Meteorological Satellite Program (DMSP). A total of 3247 four-sheet events were identified by applying an automatic procedure to the magnetic field data. The occurrence frequency of the events was less than 10% in each magnetic local time (MLT) sector for both hemispheres. The selected events were classified into two groups (type W and type M) with respect to the polarity of FACs. The polarity of the most equatorward type W (type M) FAC sheet is upward (downward). Type W (type M) events are observed predominantly in the morning (evening) sector and display a clear interplanetary magnetic field (IMF) BY dependence. Type W (type M) events appear for negative (positive) IMF BY in the Northern Hemisphere with the opposite dependence in the Southern Hemisphere. For each FAC sheet for type W events, we examined the occurrence of different precipitation regions identified with another automatic procedure. The most equatorward FAC sheet is collocated predominantly with central plasma sheet precipitation except for the prenoon sector, where boundary plasma sheet (BPS) precipitation has the highest occurrence. For the second and the third equatorward sheets, boundary layer-like precipitation, BPS near dawn and low-latitude boundary layer near noon, is dominant. In the most poleward sheet, mantle precipitation was observed frequently near noon, while BPS precipitation was dominant at earlier MLTs. These results, especially precipitation characteristics near noon, are consistent with our interpretation of the four-sheet structures, which is based on IMF BY dependence of convection patterns consisting of lobe, merging, and viscous cells.

[1] 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(A c )), 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 dΦ Mp / dt = v 4/3 B T 2/3 sin 8/3 (θ c /2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin 8/3 (θ c /2))(interplanetary field magnitude, B T )(merging line length, ∼(B MP /B T ) 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 B T and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dΦ MP /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), dΦ MP /dt accounts for about 57.2% of the variance, compared to 50.9% for E KL 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 d ΦMP /dt is Dst, which correlates best (r = 0.87) with p 1/2 dΦ MP /dt. If dΦ MP /dt were credited with this success, its average score would be even higher.

The oldest documented1,2 relationship between the number of sunspots (the solar cycle) and terrestrial effects is the increased frequency of aurorae in the period immediately after the solar maximum (the peak of the number of sunspots). This correlation is, however, based only on observations of the relatively rare events of ‘great aurorae’, which are those that reach mid-latitudes or lower. The overwhelming majority of intense aurorae, and therefore most of the energy put into the ionosphere, occurs at high latitudes, where aurorae appear nightly. Here we report the global frequency of aurorae as a function of solar cycle, determined by data from the US Air Force Defense Meteorological Satellite Program. We find that, contrary to expectations, the total number of intense aurorae is uncorrelated with solar activity in darkness, and is negatively correlated with solar activity in sunlit conditions. These findings imply a causal relationship between aurorae and ionospheric conductivity (the latter is maximal at solar maximum) and therefore indicate that the occurrence of intense aurorae is a discharge phenomenon, similar to lightning.

[1] 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.

[1] 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 (EKL = vBTsin2(θc/2), where θc 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 Bz function used in previous work, and also better than the ɛ parameter (ɛ = vB2sin4(θc/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 Bz > 0) and the Kan-Lee function. Namely, EWAV = vBTsin4(θc/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 EWAV 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.

[1] During periods of northward IMF, plasma sheet ions often have two components: hot (magnetospheric origin) and cold (magnetosheath origin). The temperatures of the cold-component ions are ∼30–40% higher in the dawn sector compared to the dusk sector, implying the dawnside magnetosheath ion heating of ∼30–40%. As a result, the magnetosheath ions are less distinguishable from the hot-component ions, which have lower temperatures on the dawnside, leading to a higher occurrence of the ions having (apparent) one-component distribution. As the duration of the hourly averaged IMF being northward (Δt) increases from 1 to 10 hours, the occurrence of two-component ions increases from 65% to 83% in the dusk flank, but in the dawn flank it remains relatively stable at around 45%. In contrast, the occurrence of ions best characterized by kappa (κ) distribution increases from 25% to 35% in the dawn flank whereas in the dusk flank it remains relatively insensitive to Δt (10%). The occurrence of a one-component Maxwellian distribution appears to be most pronounced in the region of the plasma sheet close to the midnight meridian, and these ions appear to be characteristic of the nominal plasma sheet (hot component) ions. The densification of the plasma sheet, as Δt increases, mainly results from the influx of the magnetosheath ions. However, the cooling of the plasma sheet ions can be attributed not only to the influx of the cold magnetosheath ions but also to the cooling of the nominal plasma sheet ions. The dawn-dusk asymmetries observed in the cold magnetosheath ion profiles should provide constraints that can help determine the roles of various proposed magnetosheath ion entry mechanisms.

The Cassini spacecraft in orbit around Saturn has observed electron beams accelerating away from the planet in regions that map to the locations of Saturn's aurora. The conventional model for aurora creation involves electron beams that accelerate towards a planet's surface. A similar phenomenon was recently observed on Earth, suggesting that electrons going the ‘wrong’ way are a general property of aurora, and that this characteristic needs to be incorporated into any full theory of aurora formation. Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet1. Surprisingly, electrons accelerated in the opposite direction have been recently observed2,3,4,5,6. The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth2,3,4,5,6 and the barely understood electron beams in Jupiter's magnetosphere7,8, demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.

[1] This paper reports results from a detailed analysis of theta auroras that occurred during the passage of a magnetic cloud on 8 November 2000. The interplanetary magnetic field (IMF) was generally northward for more than 12 hours, while the y-component of the IMF changed signs several times. Auroral images of the Northern Hemisphere acquired from the Ultraviolet Imager (UVI) on board the Polar satellite showed clear development of theta auroras during the prolonged northward IMF period. It is found that the theta aurora can be enhanced by compression of the magnetosphere associated solar wind pressure pulses. We also examine large-scale ionospheric plasma convection flow data from the SuperDARN radar network to study the ionospheric plasma flow pattern associated with the theta aurora. The radar data, when available, showed that two different plasma flows can coexist within the day-night part of the theta aurora. The nightside extension of the theta aurora is always on a region of antisunward convecting flows, whereas there is no consistent flow pattern found in the dayside portion of the theta aurora studied. In general, plasma flow outside the theta aurora cannot be determined because of lack of scattered radar signals, but from a few instances it suggested that the nightside theta aurora is flanked by antisunward flows. In contrast to the prevailing view that theta auroras reside in regions of closed magnetic flux that convect sunward, this study suggests one more aspect of the theta aurora and provides constraints for models and theories.

[1] The coincident TIMED/Global Ultraviolet Imager (TIMED/GUVI) optical and DMSP particle observations have revealed new features of the optical signature of the polar cusp under a southward interplanetary magnetic field (IMF). We have found that cusp auroras usually take the shape of a thin arc with a width around 100–200 km. This provides the first far-ultraviolet evidence of the narrow cusp under a southward IMF [Newell and Meng, 1987]. The cusp auroras could extend down to 0800 magnetic local time (MLT) in the morningside and 1400 MLT in the duskside. Its length is about a few thousand kilometers. A large solar wind density, speed, and IMF are necessary conditions for GUVI to observe the cusp aurora. We found that the cusp location at 1200 MLT changes linearly (−10 nT Bz < 0 nT) and nonlinearly (Bz < −10 nT) with the IMF Bz. The nonlinear effect can be explained by an L−3 dependence of the Earth's equatorial magnetic field.

[1] Auroral boundary algorithms for SuperDARN ionospheric convection observations are introduced. Both the equatorward boundary of scatter (EBS) and the convection reversal boundary (CRB) are found and compared with boundaries based on DMSP particle precipitation observations. These comparisons provide an intercalibration between the two schemes and insight into the relationship between disparate phenomena. The EBS is reliably poleward of the equatorward boundary of precipitation (EBP) at most local times but centers well on the equatorwardmost precipitation between 1600 and 2200 MLT when within 8° latitude of the radar station, indicating a correspondence with the SLERPS (slow long-lived E region plasma structures) of Jayachandran. Near dawn the EBS coincides better with the structured–unstructured boundary (SUB) than the EBP, as one might expect if the ionospheric irregularities responsible for this scatter were produced by the gradient-drift instability. The CRB correlates well with the open-closed boundary (OCB) as seen by DMSP. There is, however, an equatorward offset of the CRB relative to the OCB that varies from zero near noon to ∼1° near dawn and dusk and is largest near midnight. This offset is consistent with a small viscous-like interaction between the magnetosheath and the low-latitude boundary layer, resulting in a degree of antisunward flow on closed field lines. The offset decreases under the influence of southward interplanetary magnetic field, as one might expect if increased dayside merging interfered with this effect.

Cusp properties during periods of By dominant IMF are investigated, 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 a 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∣.

[1] The temporal variation of the equatorward boundary of the proton aurora/high-energy ion precipitation is a manifestation of diurnal and seasonal (i.e., dipole tilt) effects as well as magnetic activity. In particular, during the substorm growth phase this boundary moves equatorward, an effect due primarily to thinning and earthward motion of the cross-tail current in the inner magnetosphere as the field evolves toward a more stretched topology. Recent advances in monitoring this boundary using ground-based instruments have opened up the possibility of following its temporal evolution across several hours in local time. This in turn allows one to explore whether this magnetotail stretching is a global or local phenomenon. We have examined this boundary evolution during the growth phases of 68 substorms over the Canadian sector. We use the equatorward boundary of SuperDARN E region echoes as a proxy for the proton auroral boundary as described by Jayachandran et al. (2002b). We find that in 21 of the 68 substorms the equatorward motion of the auroral boundary is restricted to several hours of local time in the evening sector. In the remaining 47 substorms, the equatorward motion was global so that the boundary retained its shape throughout the growth phase. Our results indicate dramatically different growth phase phenomenology in these two classes of substorms. In one, the growth phase involves stretching in the inner magnetosphere that is most pronounced around the onset meridian. In the other, the stretching extends many hours in local time away from the onset meridian.

[1] Recent studies have shown that intense discrete aurora, auroral kilometric radiation, upflowing ion beams, and downward directed electric fields are more intense in the winter hemisphere than in the summer. This is particularly true of the dusk to midnight sector where intense electron aurora are most common. Here we use one solar cycle of DMSP satellite particle data to investigate the seasonality of the ion aurora. The ion aurora proves to be approximately equal in the summer and winter hemispheres in the dusk-midnight sector (with the summer hemisphere favored by 0 to 4%). However, in the MLT hours from midnight to dawn, the ion precipitating energy flux is 15–40% higher in winter than in summer. The absolute magnitude of the ion effect is smaller than was found for discrete electron aurora (which show a threefold difference between winter and summer). The seasonal behavior of the ions may reflect the observation that diverging electric fields, which accelerate ions downward, are found mainly postmidnight, and are stronger in the winter. The relative weakness of the seasonal effects in ions may reflect their high average energy (many tens of keV), which is substantially larger than typical of electric potentials found in the auroral circuit. Ions in the dusk to midnight sector are most intense equatorward of the region of discrete aurora, and are thus probably not much affected by the seasonality of the field-aligned electric fields which exist there (and which are of the sense to retard ion precipitation). Interestingly, ion average energies are higher in the winter hemisphere than in the summer hemisphere at all local times, regardless of whether or not energy fluxes are enhanced. The above results apply to the actual range of ion energies measured by the DMSP electrostatic analyzers (32 eV to 30 keV). A sample extrapolation to 100 keV energy, assuming the spectra are Maxwellian, showed the same overall pattern. The percentage by which the winter hemisphere was favored over the summer for precipitating ion energy flux was moderately larger when using extrapolation, and a larger difference was still seen postmidnight than premidnight.

[1] Eleven years of DMSP particle data (1984–1994) were used to investigate electron acceleration events on the dayside. Specifically, the characteristic energy (spectral flux peak), total energy flux, and probability of observing acceleration events were studied as a function of the type of dayside precipitation region (cusp, mantle, etc.). Most dayside electron acceleration events are latitudinally smaller than the events found premidnight, so it is usually possible to identify the type of precipitation which surrounds dayside events. All dayside regions contain embedded electron acceleration events, with such instances most common in the dayside boundary plasma sheet (BPS) and least common in the mantle and polar rain. Electron acceleration events in the cusp are not particularly rare, but the acceleration involved is quite mild. This is consistent with findings that large nightside potentials do not form in regions with a large ambient electron population. On the basis of the fraction of spectra which are accelerated (number of accelerated spectra divided by the total number of spectra measured), the dayside regions can be ordered as follows, with the acceleration probability in parenthesis: mantle prenoon (2.4%), mantle postnoon (3.0%), cusp (4.2%), open low-latitude boundary layer (LLBL) (5.0%), closed LLBL prenoon (6.0%), closed LLBL postnoon (7.2%), BPS prenoon (8.2%), and finally BPS postnoon (8.6%). The total precipitating energy flux from electron acceleration has the same order; thus more dayside auroral luminosity is associated with the BPS than with any other region. When these regions are ordered by the spectral flux peak in acceleration events, the order changes, as follows: cusp (201 eV), open LLBL (234 eV), closed LLBL prenoon (390 eV), mantle prenoon (414 eV), closed LLBL postnoon (459 eV), mantle prenoon (463 eV), BPS prenoon (572 eV), and BPS postnoon (725 eV). The afternoon hot spot is found to have a relatively symmetric partner, a morningside “warm spot,” centered near 0830 MLT. Both the afternoon hot spot and the morning warm spot span the convection reversal boundary, since they include both BPS and LLBL precipitation, with the former region showing the most intense activity (both the highest number of events and the largest electron acceleration energies).

[1] Precipitating particle data from the DMSP F12 and F13 satellites has been merged with image data from the Ultraviolet Imager (UVI) on the Polar satellite. UVI images were combined within single transpolar passes of the DMSP F12 and F13 satellites. Pixel irradiances were then averaged into 1° × 1° bins along the satellite tracks, thus generating along-track irradiance profiles. Similarly, energy fluxes of precipitating electrons were averaged into 1° bins along the trajectory. The peaks in each set of profiles were isolated in order to compare photon irradiance with electron energy flux. The irradiance and energy flux data of hundreds of peaks were linearly correlated for January–February 1997 when the northern polar regions were in darkness and dayglow effects were minimal. In the pre-midnight and dawn sectors, good correlations (r > 0.7) arise between the photon irradiance and electron energy fluxes, while correlations for other sectors are not as good. The good correlation suggests that UVI fluxes can be directly calibrated to measure energy input without the additional complication of modeling, at least for some MLT sectors. Using this calibration, global maps of energy input into the aurora may be derived empirically.

An intense dayside proton aurora was observed by Imager for Magnetopause-to- Aurora Global Exploration Far Ultra-Violet imager (IMAGE FUV) for an extensive period of northward interplanetary magnetic field (IMF) on 17 and 18 September 2000. This aurora partially coincided with the auroral oval and intruded farther poleward into the polar cap, and it showed longitudinal motions in response to IMF By variation. Intense magnetosheath-like electron and ion precipitations have been simultaneously detected by Defense Meteorological Satellite Program (DMSP) above the poleward portion of the high-latitude dayside aurora. They resemble the typical plasmas observed in the low-altitude cusp. However, less intense electrons and more energetic ions were detected over the equatonvard part of the aurora. These plasmas are closer to the low-latitude boundary layer (LLBL) plasmas. Under strongly northward IMF, global ionospheric convection derived from Super Dual Auroral Radar Network (SuperDARN) radar measurements showed a four-cell pattern with sunward convection in the middle of the dayside polar cap and the dayside aurora corresponded to two different convection cells. This result further supports two source regions for the aurora. The cusp proton aurora is on open magnetic field lines convecting sunward whereas the LLBL proton aurora is on closed field lines convecting antisunward. These IMAGE, DMSP, and SuperDARN observations reveal the structure and dynamics of the aurora and provide strong evidence for magnetic merging occurring at the high-latitude magnetopause poleward from the cusp. This merging process was very likely quasi-stationary.

[1] We present maps of ionospheric precipitation regions, based on 11 years of DMSP particle data, binned by the interplanetary magnetic field (IMF), with superposed SuperDARN convection streamlines gathered under similar conditions. The convection patterns are transformed into an inertial coordinate system. The maps, which include both the nightside and dayside, are created in a fully automated fashion, with, for example, the cusp centered at its centroid latitude for each half hour bin of MLT, with a latitudinal width equal to the statistical difference between the poleward and equatorward edges. The mantle asymmetry about noon does not fit the pattern expected from simple theoretical considerations (namely, that the mantle should be thicker postnoon for positive By in the Northern Hemisphere). The mantle is appreciably thicker prenoon than postnoon, especially for positive By but also even for negative B y This asymmetry matches the SuperDARN convection flows, in which, irrespective of the sign of B y , most of the conversion of closed field lines to open occurs prenoon. Quantitatively expressed, for southward IMF, the potential encompassed by flux crossing the open-closed boundary prenoon (0600-1200 MLT) exceeds that for postnoon (1200-1800 MLT) by 30 kV to 15 kV for By > 3 nT and by 30 kV to 20 kV for By 3 nT, 31 kV reconnects from 1800 to 2400 MLT, but only 14 kV reconnects from 0000 to 06000 MLT. The convection reversal boundary (CRB) consistently coincides with the nightside open-closed particle boundary (OCB). On the dayside, the CRB lies equatorward (poleward) of the OCB in the postnoon (prenoon) sector for B y 0). This shift is consistent with the effects of an interhemispheric current produced by the partial penetration of the IMF By into the frontside magnetosphere.

[1] There has been an impression for more than half a century that reductions in the north-south component of the Earth's magnetic field in the polar region (often called “negative magnetic bays” or “polar magnetic bays”) may occur immediately after compression of the magnetosphere by interplanetary (IP) shock impacts, in particular when the magnetosphere is “preconditioned” by southward interplanetary magnetic field (IMF). A literature search suggests that this view has not been rigorously verified nor disproved. This paper reports a study of 43 IP shock events to illustrate the effect of IP shock-magnetosphere coupling on the high-latitude auroral electrojets. Specifically, we correlate the strength of magnetic bays (inferred from minimum values of the AL index within a 30-min window after shock impact) with solar wind parameters and their combinations within the same time window. It is surprisingly found that the strength of magnetic bays correlates much better with solar wind parameters downstream of a shock (correlation coefficient, r, up to 0.86) than solar wind parameters upstream of a shock (r up to 0.6). Therefore whether or not an interplanetary shock can “induce” a magnetic bay depends on the concurrent solar wind and IMF conditions. This strongly suggests that enhancements of the westward auroral electrojet during magnetosphere compression are primarily associated with a directly driven process. It is also found that the solar wind dynamic pressure, which is traditionally considered a secondary effect, contributes equally with IMF Bz to the westward auroral electrojet.

Auroral particles have been studied for years using DMSP satellites. The goal of this project was to apply past research to develop practical tools for monitoring the real-time position of the auroral oval (nowcasting), and more generally, monitoring the general state of space weather from DMSP particle data. The effort was successful. A real-time auroral oval monitor (OVATION) was developed, and transitioned to AF space weather operations. The OVATION effort nowcasts the current auroral oval position and intensity, along with the general state of excitation of the near-Earth space conditions ("space weather"). The tools developed in creating OVATION in turn provided new opportunities for basic research. One important finding which resulted is that a seasonal variation in space weather intensity exists, which depends primarily on whether the nightside auroral oval of either the northern or southern hemisphere is sunlit.

[1] The triggering mechanism(s) for the substorm expansion phase onset is one of the outstanding issues in magnetospheric physics. Previous studies have shown that an impingement of the high solar wind dynamic pressure may lead to the expansion phase onset of a substorm. These previous studies typically used a negative magnetic bay as the main proxy for a substorm, but we now know that some magnetic bays are associated not with substorms but with the enhancement of convection. Therefore it is reasonable to cast doubts on the compression trigger mechanism. In this study we use classical substorm onsets, “auroral breakups,” which are the most reliable substorm onset indicator when identified with global auroral images, to reinvestigate this issue. We examine 43 interplanetary shock events that occurred between 1996 and 1999 with simultaneous global auroral images from the Polar ultraviolet imager images and the auroral electrojet indices. It is found, indeed, that ∼52% of the shocks produce magnetic bays (AL < −100 nT). While most of the shocks enhance auroral luminosities, only 4 events (∼9%) appear to have an auroral breakup preceded by an SSC/SI by 20 min (two events by 10 min). These results strongly indicate interplanetary shocks can produce negative magnetic bays but not auroral breakups (thus christened “compression bays”). An examination of precisely timed interplanetary magnetic field (IMF) during 11 substorms that occurred near the shock-induced SSCs/SIs indicates that northward turnings of IMF occurred more than 50% of time. Given such a low probability, it is concluded that shock compression is not likely to trigger substorms but enhances magnetospheric currents and auroral particle precipitation. In line with many previous study results, on the other hand, the northward turnings of IMF can be a plausible substorm triggering mechanism.

[1] Equatorward and poleward auroral boundaries from latitude profiles of Polar Ultraviolet Imager (UVI) auroral images were correlated with over 23,000 boundaries derived from 2 years of Defense Meteorological Satellite Program (DMSP) electron precipitation data. Latitude differences between DMSP and UVI boundaries were averaged into 1-hour and 3-hour sectors of magnetic local time (MLT). The statistical distributions of these differences generally peak near zero but have Gaussian-like shapes with widths of about 3°–4° in magnetic latitude. The mean values of these offsets exhibit systematic trends in MLT, with the largest disagreement near 05 MLT for the poleward boundaries and near noon for the equatorward boundaries. The mean offsets were fit to second-order harmonic expansions, which approximately “calibrate” image boundaries with respect to the precipitating electron boundaries. The harmonic fits suggest that the images can yield approximate precipitation boundaries for such purposes as estimating the area of the polar cap.

[1] Magnetic conjugate observations by Cluster and DMSP F14 satellites are used to study the field lines of auroral arc. Cluster is well above the acceleration region and observes upward keV ion beams and bipolar electric structures. The integrated potential at Cluster altitudes shows a dip that is consistent with the keV electron acceleration energy at low altitude. The earthward Poynting flux at Cluster altitudes is comparable to the electron energy flux at low altitudes. Thus, for this event the auroral acceleration can be described as a quasi-stationary potential structure with equipotential lines reaching the Cluster altitudes. The arc forms at the outer edge of the plasma sheet at a density gradient. Multiple Cluster satellite measurements allow us to study the density increase associated with the development of the arc, and to estimate the velocity of the structure. The quasi-potential structure itself may be part of an Alfven wave.

[1] In this study we quantitatively investigate auroral power rate of change during periods of plasma sheet fast flows. We first identify plasma sheet fast flows from Geotail plasma and magnetic field data and then estimate the rate of change in integrated auroral power over the nightside auroral region using Polar Ultraviolet Imager auroral images over the course of the fast flows. It is found that almost all tailward fast flows are associated with increasing auroral power. Half of earthward fast flows are associated with decreasing auroral power, indicating that the earthward fast flows are not necessarily associated with increasing auroral power. In addition, we quantitatively investigate relationships between the integrated nightside auroral power rate of change and average local magnetic field Bz at Geotail during fast flow periods. We find that almost all negative Bz events are associated with increasing auroral power. The integrated auroral power for positive Bz events increases and decreases almost equally often. Results derived from a further division of the earthward fast flows events at X = −20 RE show that 67% of the earthward fast flows for X >−20 RE are associated with increasing auroral power, but 54% of the earthward fast flows for X < −20 RE are associated with decreasing auroral power. Moreover, percentage of earthward fast flows with increasing auroral power for the 2300–0100 MLT sector is significantly larger than those for other two local time sectors (2100–2300 and 0100–0300 MLT).

We have developed a technique for simultaneous comparison of the ULF (ultra-low-frequency) activity in the Pc5 frequency band, global patterns of field-aligned currents (FAC), derived from the IZMIRAN Electrodynamics Model (IZMEM) driven by the interplanetary magnetic field, and the ionospheric projections of the dayside magnetospheric domains derived from DMSP satellite particle data. This technique produces a sequence of two-dimensional snapshots of the FAC distributions with overlapped DMSP tracks, and the ULF spectral power based on data from a worldwide array of geomagnetic stations. Several events have been analysed as examples. In the cases considered we have found that in the morning sector the IZMEM-predicted region of downward FAC maps together with the low-latitude boundary layer (LLBL) projection to the polar ionosphere, whereas the upward FAC region coincides with the central plasma sheet (CPS) projection. The simultaneous occurrence of two sources of ULF activity in the nominal Pc5 frequency band has been identified: one source is located in the dawn sector at geomagnetic latitudes 65°-70°, and another source is located near noon at magnetic latitudes 75° - 79°. The peak of Pc5 in the morning sector is situated equatorward of the Region 1 FAC system, in the region of the Region 2 system or between them. The latitude of the near-noon Pc5 pulsations peak power coincides with the equatorward boundary of the low latitude boundary layer, whereas the resonant maximum of Pc5 pulsations in morning hours corresponds to the CPS region.

[1] We compare auroral power (AP) as derived from a global imager with the polar cap (PC) index for the periods of December 1996 through January 1997 (winter) and June 1997 through July 1997 (summer). The auroral power is inferred from auroral luminosity in the N2 Lyman-Birge-Hopfield bands (λ ∼ 160–180 nm) acquired from the ultraviolet imager on board the ISTP/GGS program's Polar satellite in the Northern Hemisphere. The PC index (1-min time resolution) is based on the horizontal component of the magnetometer recordings from a single near-geomagnetic polar station at Thule (85.4° corrected geomagnetic latitude). A fair correlation exists between the hemispheric auroral power and PC (r = 0.70), with higher correlation for winter (r = 0.77) compared to summer (r = 0.69). Also, PC correlates with nightside auroral power much better than with dayside auroral power. The best AP-PC correlation took place in the pre-dawn and post-dusk sectors. The seasonal and diurnal effects are a result of competition between the ionospheric convection currents and field-aligned currents manifested by seasonal and diurnal variations in ionospheric conductivity. Finally, a comparison of the AP-PC correlation between substorm and non-substorm times indicates that substorm electrojets contribute little, if not at all, to the PC index.

Since the late 1970s, reports of temporal injections of sheath plasma deep inside the magnetosphere have recurred. Most reports favor northward (especially weakly so) IMF conditions, and there may be a preference for the dawn flank. Injections occur preferentially when solar wind pressure is high, and during intervals of incident pressure pulses. Many low-altitude observations lie in 0800-1000 MLT sector. High altitude observations of sheath injections into the closed magnetosphere extend downstream of dawn or dusk. Some cases initially assumed to be cusp injections are, based on the spectral properties, the local time, and other characteristics, actually in the closed LLBL. It is unclear whether these injection events are a relatively minor aspect of solar wind-magnetosphere coupling, or whether they play a significant role, say, in LLBL formation. Nonetheless, the evidence for sheath plasma entering onto closed field lines by processes other than merging is strong.

[1] We compare the timing and spatial extent of dipolarization with auroral breakup by using magnetic field data (1-min time resolution) from GOES 8 and GOES 9 and global auroral images (37-s frame rate) from the Polar ultraviolet imager. We survey the entire year of 1997 data and obtain 32 unambiguous dipolarization events that had ionospheric footprints within 2 hours in magnetic local time (MLT) from their associated auroral breakups. It is found that the auroral breakup preceded the arrival of dipolarization at GOES by an average of 1.7 ± 2.7 min. (1.0 ± 2.1 min for events ±1 hour from the auroral breakup). The delay of geosynchronous dipolarization represents a propagation time, as the substorms rarely initiated at the GOES location. Propagation velocities derived from the propagation time are ∼60 km s−1 eastward, ∼75 km s−1 westward, and ∼270 km s−1 earthward at geosynchronous orbit. We also show for the first time that auroral bulge and dipolarization region closely map; the bulge is the region of currents out of the ionosphere. The eastern edge of the dipolarization region, where field-aligned currents are flowing into the ionosphere, is, based on the T89 magnetic field model, located at ∼0.5 hour in MLT eastward of the eastern edge of the auroral bulge. Within the initiation site meridian the dipolarization onset was observed ∼1 min prior to the auroral breakup at the GOES altitude. This time is associated with Alfven transit time and therefore depends on the radial location of the onset. The azimuthal scale of the initiation site is found to be ∼0.5 hour in MLT in the ionosphere or less than ∼1 Earth's radius at 6.6 RE. These results support current diversion into the ionosphere as a critical part of substorms. Finally, a simple timing analysis of specific substorm onset time features suggests that near-Earth reconnection cannot occur before current disruption and therefore cannot be the cause of substorm.

[1] We modeled the magnetotail configuration during a highly disturbed period on 19 April 1985 based on data from three magnetospheric spacecraft (Active Magnetospheric Tracer Explorers (AMPTE)/IRM, AMPTE/CCE, and ISEE 1) and proton precipitation measured by DMSP spacecraft. The goal was to test the procedure of including the plasma pressure as input data when constructing the magnetotail magnetic field model. On the basis of AMPTE/IRM data we confirm statistically a high degree of pressure isotropy at 8–17 RE distances in the tail plasma sheet, which allows computation of the plasma pressure from magnetospheric magnetic field model and easy mapping of the pressure along the corresponding flux tubes. The usage of pressure data not only increases the amount of data for event-oriented modeling but also imposes the nonlocal constraints on the configuration and mapping between magnetosphere and ionosphere to stabilize the solution of inverse problem and get it to be more realistic. Observational and modeling results indicate large systematic deviations of observed configuration from predictions of standard models (T89 or T96) [Tsyganenko, 1989, 1995] and reveal highly variable configurations, justifying a need for the development of event-oriented modeling. Indications of localized B minimum and current density maximum at around 12 RE have been obtained in the model with best data coverage, but precipitation from this region showed no signatures of discrete structures. The structured precipitation has been mostly seen in poleward half of the auroral zone precipitation, which is mapped to rather distant parts (>30 RE) of the plasma sheet.

[1] We present several findings that improve the understanding of the seasonal and diurnal variation in auroral and magnetospheric activity. The total ionospheric conductivity in the nightside auroral oval from UV insolation (ΣP) is calculated, and its seasonal and diurnal variation is shown to correlate very highly with that of the Am and AL indices of geomagnetic activity (r = 0.89 and r = 0.75, respectively). Such excellent correlations with Am have been previously obtained by other researchers using instead the acute angle between the Earth's dipole axis and the Earth-Sun line, ψ. However, the ionospheric conductivity formulation provides a more physical model to explain the equinoctial (McIntosh) effect. Namely, the level of geomagnetic activity is well-ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither. We improve calculations of the expected pattern of seasonal and diurnal variations in the solar wind input. The elliptical nature of the Earth's orbit results in observed interplanetary magnetic field (IMF) strengths about 7% larger in January than June. When the Sun's spin axis tilt to the ecliptic plane is considered, the predicted IMF southward component (Bs) maximizes in February, as is observed. We also calculate the seasonal and diurnal variation of a more general solar wind–magnetosphere coupling function, EKL. EKL proves to have little (0.5%) diurnal variation and has a seasonal variation of about 14%. For the first time, the seasonal and diurnal variation in the ΦPC, the polar cap flux (from Polar UVI observations, cross-calibrated to a DMSP-based standard) and in magnetotail stretching (the b2i index) are presented. Magnetotail stretching proves to correlate better (r = −0.57) with EKL than with ΣP. ΦPC correlates better with ΣP, but the correlation (r = −0.49) is not nearly as strong as that for the indices of geomagnetic activity, Am and AL. Our survey of the seasonal and diurnal variation of the magnetosphere thus shows that some aspects (geomagnetic indices) correlate best with UV insolation, while others (magnetotail stretching) correlate best with solar wind input.

The origin of the diffuse aurora, whose beauty and intensity pale beside those of the famous aurora borealis, has remained controversial. A convincing explanation for this auroral display is now at hand. See Letter p.943 Earth's diffuse aurora, almost invisible from the ground, was discovered in images from the Isis-2 satellite in 1971 as a broad ring of light around the auroral oval. The diffuse aurora is the main source of energy for the high-latitude night-side upper atmosphere, and for many years there has been controversy over the mechanism causing the scattering loss of plasma-sheet electrons that powers it. Two distinct classes of magnetospheric plasma waves have been suggested: electrostatic electron cyclotron harmonic waves, and the more structured whistler-mode 'chorus' waves generated by electrons travelling through Earth's magnetic field. Thorne et al. report an analysis of satellite wave data that reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation.

Large-scale field-aligned current (FAC) systems occasionally disappear in the midday sector (9 < MLT < 13), even though this is the local time sector where FACs tend to be most intense. The present study investigates 26 such events observed by the DMSP-F7 satellite. It is found that the events tend to occur in the winter hemisphere in a certain UT range when the dipole axis was inclined most antisunward and therefore the contribution of solar illumination to the ionospheric conductivity is minimum. Before most events, IMF B Z was positive, and the solar wind bulk flow momentum was below the average. Although one might expect to see such events when the sign of B X was unfavorable for merging at the high-latitude tail magnetopause in the winter hemisphere, no clear preference for IMF B X was found. It was also found that the asymmetry of the ionospheric conductivity between the summer and winter hemispheres suppresses the IMF-dependent characteristics of particle precipitation in the polar cap. These results suggest that ionospheric conductivity not only determines the intensity of FACs for the convection electric field imposed by the solar wind-magnetosphere interaction, but also affects how the magnetosphere interacts with the solar wind.

This paper presents results from a study of one year’s data from the recently installed Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada and from two stations of the Automated Geophysical Observatories (AGOs) in Antarctica. The magnetometer data from these conjugate arrays were used to study ULF waves in the Pc 1–2 (100–600 mHz) frequency band at cusp and polar cap latitudes (74° – 80° invariant latitude). In this paper we focus on the differences between observations in different hemispheres, spectral properties and latitudinal - local time distributions of Pc 1–2 events observed during 1994. The latitudinal trends and case studies are used to infer the source locations of the two major wave types we have observed. Broadband waves, with diffuse spectral character, were more prevalent at the higher latitudes. Waves with narrower bandwidth were much more common in our data set, and were the statistically dominant wave type at all the MACCS stations. These multistation observations, combined with data from DMSP satellite overpasses, suggest the possibility that these two wave types originate in quite different regions near the magnetospheric boundary; the more narrowband waves in the subsolar and postnoon equatorial region, and the more broadband waves in the high latitude plasma mantle (and possibly at the poleward edge of the cusp). The case studies also reveal a connection between periods of IMF Bz northward and cutouts in broadband Pc 1–2 wave power, and hemispherical differences in Pc 1–2 observations are interpreted in terms of dipole tilt angle effects.

We review and summarize the evidence that a closed LLBL exists, at least away from noon. We also emphasize the observation that just equatorward of the LLBL, overlapping magnetosheath and magnetospheric electrons exist on sunward convecting field lines which have no low-energy ion cutoff (i.e., closed field lines). Therefore it is inescapable to conclude that sheath plasma is introduced onto closed field lines. The dropoff of the high-energy electrons is identified not necessarily with the open/closed boundary but rather with the convection reversal boundary, since these electrons originate on the nightside and convect towards the dayside (hence they cannot exist on anti-sunward convecting field lines, even if the latter are closed). A model is discussed wherein diffusion introduces sheath plasma onto closed field lines, but merging removes these same field lines from the dayside. The competition between these two effects explains why the LLBL is thinnest at noon, where open LLBL signatures dominate (perhaps exclusively), and why the LLBL is thicker for northward IMF.

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the 'snapshot' distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When B(sub y) is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

A new type of aurora has been observed. Although similar in shape to conventional aurora, it lacks their intensity and dynamics, instead rotating quietly with the Earth.

We discuss, in order, the following topics: (1) A brief history of auroral transients; (2) Perspective on the importance of such transients; (3) An alternative explanation for the transients; (4) A caution about reference frames and the Siscoe-Huang convection model; (5) A critique of the Cowley-Lockwood convection model; (6) The staircase cusp and bursty merging; and finally (7) A Weltanshauung summary of what is and isn’t established.

Earlier this year, the four satellites of the Cluster mission passed through part of the electric circuit that causes aurorae. Their observations support the view that intense aurorae form in regions largely devoid of electrons.

It has been hard to relate the activity of the northern lights to specific events in space. But the latest data show that 'auroral streamers' can be matched to bursts of ionized particles from Earth's magnetotail.