Polar UVI Research Articles

Comparison of the Ionosphere During an SMC Initiating Substorm and an Isolated Substorm

AbstractIn order to assess the effects of ionospheric feedback on different modes of energy transport in the magnetosphere, we investigate an isolated substorm and a steady magnetospheric convection (SMC) event with very similar solar wind drivers. The primary focus is on a comparison between the isolated substorm and the substorm that initiates the SMC. Auroral data from Polar UVI LBHl and LBHs, along with assimilative mapping of the ionosphere electrojet potential patterns are used as inputs to the global ionosphere‐thermosphere model to calculate conductances and Joule heating rates. Results from this study show that the conductances both before and during the events play a large role the ability of the magnetosphere to remain in steady driven state. The substorm that initiates the SMC event shows very different signatures in the ionosphere than isolated substorm; these signatures indicate that there is very weak substorm current wedge, or possibly a pseudo‐breakup.

The statistical difference between bending arcs and regular polar arcs

AbstractIn this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary magnetic field (IMF) By direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arc's nightside end remains attached to the oval, eventually becoming hook‐shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during By‐dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90° about 20 min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF By and initial arc location. In our data set, the highest correlations are found with IMF By appearing at least 1–2 h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection.

Electron precipitation models in global magnetosphere simulations

AbstractGeneral methods for improving the specification of electron precipitation in global simulations are described and implemented in the Lyon‐Fedder‐Mobarry (LFM) global simulation model, and the quality of its predictions for precipitation is assessed. LFM's existing diffuse and monoenergetic electron precipitation models are improved, and new models are developed for lower energy, broadband, and direct‐entry cusp precipitation. The LFM simulation results for combined diffuse plus monoenergetic electron precipitation exhibit a quadratic increase in the hemispheric precipitation power as the intensity of solar wind driving increases, in contrast with the prediction from the OVATION Prime (OP) 2010 empirical precipitation model which increases linearly with driving intensity. Broadband precipitation power increases approximately linearly with driving intensity in both models. Comparisons of LFM and OP predictions with estimates of precipitating power derived from inversions of Polar satellite UVI images during a double substorm event (28–29 March 1998) show that the LFM peak precipitating power is >4× larger when using the improved precipitation model and most closely tracks the larger of three different inversion estimates. The OP prediction most closely tracks the double peaks in the intermediate inversion estimate, but it overestimates the precipitating power between the two substorms by a factor >2 relative to all other estimates. LFMs polar pattern of precipitating energy flux tracks that of OP for broadband precipitation exhibits good correlation with duskside region 1 currents for monoenergetic energy flux that OP misses and fails to produce sufficient diffuse precipitation power in the prenoon quadrant that is present in OP. The prenoon deficiency is most likely due to the absence of drift kinetic physics in the LFM simulation.

OVATION Prime‐2013: Extension of auroral precipitation model to higher disturbance levels

AbstractOVATION 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 < dΦMP/dt ≤ 3.0 MWb/s (Kp about 8+). Other upgrades include a reduced susceptibility to salt‐and‐pepper noise, and smoother interpolation across the postmidnight data gap. The model is tested against an independent data set of hemispheric auroral power from Polar UVI. Over the common range of validity of OP‐2010 and OP‐2013, the two models predict auroral power essentially identically, primarily because hemispheric power calculations were done in a way to minimize the impact of OP‐2010s noise. To quantitatively demonstrate the improvement at high disturbance levels would require multiple very large substorms, which are rare, and insufficiently present in the limited data set of Polar UVI hemispheric power values. Nonetheless, although OP‐2010 breaks down in a variety of ways above Kp = 5+ or 6−, OP‐2013 continues to show the auroral oval advancing equatorward, at least to 55° MLAT or a bit less, and OP‐2013 does not develop spurious large noise patches. We will also discuss the advantages and disadvantages of other precipitation models more generally, as no one model fits best all possible uses.

Interhemispherical asymmetry of substorm onset locations and the interplanetary magnetic field

[1] The auroral substorm display in the conjugate hemispheres offers a unique tool to understand how the Earth's plasma and magnetic environment respond to changes in the solar wind and the interplanetary magnetic field (IMF). Earlier studies have demonstrated that substorm onset locations in the two hemispheres are systematically displaced due to the orientation of the IMF, but it is still a controversy which IMF parameter is most important. We have analysed more than 6600 substorms identified from global auroral images by Polar UVI from years 1996–2000 plus 2007 and IMAGE FUV from years 2000–2005. We find very strong statistical support for earlier conjugate auroral imaging observations, according to which the IMF clock angle, θc, organizes the average substorm onset locations in both hemispheres. The IMF θc control is a manifestation of dayside/lobe reconnection geometry and magnetic tension on open field lines before tail reconnection resulting in closed field lines with asymmetric footpoints for all θc angles. By organizing the average substorm locations by the IMF By component only we also find statistical significance. The relation is not linear, as reported earlier, but reveals saturation effects that can be explained by the non-uniform penetration of IMF By into the closed magnetosphere.

Solar wind driving and substorm triggering

[1] We compare solar wind driving and its changes for three data sets: (1) 4861 identifications of substorm onsets from satellite global imagers (Polar UVI and IMAGE FUV); (2) a similar number of otherwise random times chosen with a similar solar wind distribution (slightly elevated driving); (3) completely random times. Multiple measures of solar wind driving were used, including interplanetary magnetic field (IMF) Bz, the Kan-Lee electric field, the Borovsky function, and dΦMP/dt (all of which estimate dayside merging). Superposed epoch analysis verifies that the mean Bz has a northward turning (or at least averages less southward) starting 20 min before onset. We argue that the delay between IMF impact on the magnetopause and tail effects appearing in the ionosphere is about that long. The northward turning is not the effect of a few extreme events. The median field shows the same result, as do all other measures of solar wind driving. We compare the rate of northward turning to that observed after random times with slightly elevated driving. The subsequent reversion to mean is essentially the same between random elevations and substorms. To further verify this, we consider in detail the distribution of changes from the statistical peak (20 min prior to onset) to onset. For Bz, the mean change after onset is +0.14 nT (i.e., IMF becomes more northward), but the standard deviation is σ = 2.8 nT. Thus large changes in either direction are common. For EKL, the change is −15 nT km/s ± 830 nT km/s. Thus either a hypothesis predicting northward turnings or one predicting southward turnings would find abundant yet random confirming examples. Indeed, applying the Lyons et al. (1997) trigger criteria (excluding only the prior requirement of 22/30 min Bz < 0, which is often not valid for actual substorms) to these three sets of data shows that “northward turning triggers” occur in 23% of the random data, 24% of the actual substorms, and after 27% of the random elevations. These results strongly support the idea of Morley and Freeman (2007), that substorms require initial elevated solar wind driving, but that there is no evidence for external triggering. Finally dynamic pressure, p, and velocity, v, show no meaningful variation around onset (although p averages 10% above an 11 year mean).

Predictive ability of four auroral precipitation models as evaluated using Polar UVI global images

[1] Auroral precipitation models have been valuable tools for several decades, but it has been difficult to estimate their objective accuracy. The use of global UV imagers, which make relatively instantaneous estimates of hemispheric auroral power, provides one approach forward. We present the first such validation and quantitative comparison of auroral precipitation models. Specifically, we correlated Polar UVI images with the predictions of four precipitation models. These are the Hardy Kp model, the Brautigam IMF-based model, the Evans nowcast model currently used at NOAA, and OVATION Prime, recently introduced by Newell and colleagues. Because calibration uncertainties exist for all particle detectors, and for imagers as well, we focus on correlation coefficients rather than the absolute magnitudes. To minimize dayglow, the nightside precipitating power (1800–0600 MLT) is considered and only for cases where that entire region is within the Polar UVI field of view. Also, only instances where each model has a prediction are considered (i.e., there must be IMF data, and there must be a NOAA satellite pass within the last 1 h). Altogether, 27,613 1 min (“instantaneous”) images satisfied these criteria from 1996 to 1997. The four models investigated predict roughly half the variance in auroral power. From least to best at predicting instantaneous auroral power, the results are Brautigam IMF model (r = 0.68, r2 = 46%); Evans nowcast model (r = 0.70, r2 = 49%); Hardy Kp model (r = 0.72, r2 = 52%); and OVATION Prime IMF (r = 0.75, r2 = 56%). We also considered 1 h averages of UVI images. All four models improved, but the nowcast jumped from third to first: Brautigam IMF (r = 0.69, r2 = 48%); Hardy Kp (r = 0.74, r2 = 55%); OVATION Prime IMF (r = 0.76, r2 = 58%); Evans nowcast (r = 0.77, r2 = 59%). The nowcast approach benefits most from hourly averaging because at times more than one satellite pass is available. In principle, with enough satellites, the nowcast approach would be ideal. The advantage of OVATION Prime IMF over Brautigam IMF mirrors their respective organizing solar wind coupling functions. The reasonable performance of the Hardy Kp model, despite its 3 h cadence and limited maps, suggests that an updated model based on Kp interpolated to 1 h cadence would be competitive.

Occurrence and properties of substorms associated with pseudobreakups

We investigate how substorms with and without growth‐phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth‐phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field Em is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating Em over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much Em reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth‐phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with Em, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by Em and AE, respectively. Comparing growth‐phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth‐phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth‐phase pseudobreakups or substorms.

Multisatellite low‐altitude observations of a magnetopause merging burst

A fortuitous multisatellite observation of a dayside merging event driven by a sharp southward interplanetary magnetic field (IMF) turning illuminates several ionospheric particle and auroral reconnection signatures. Shortly after the southward IMF turning, as propagated from ACE, DMSP F13 observed a region equatorward of the cusp emptied of magnetospheric ions and bounded by two sharp intensifications of electron energy flux. The equatorward electron burst occurred at 74.4° magnetic latitude. That is also both the boundary to which magnetospheric ions were depleted and to which magnetosheath ions appeared in a DMSP F12 satellite pass 4 min later. These latter ions evinced a low‐energy cutoff, implying that about 4–5 min elapsed since merging. The equatorward electron burst observed by F13 reached an intensity of 8.6 ergs/cm2 s, and thus must have produced a strong auroral signature. Indeed, Polar UVI observed a dayside auroral transient at the appropriate latitude and local time. This burst was likely Alfvénic (it was not monoenergetic (not associated with a quasi‐static electric field)). The poleward electron burst observed by F13 (up to 4.6 ergs/cm2 s), was at the equatorward edge of the old cusp and also had a broadband (Alfvénic) signature. Drift meter data on F13 indicates that the merging burst produced flow speeds up to 2 km/s and a 47 kV cusp contribution to the polar cap potential, of which 25 kV was in a narrow high‐speed spike. Thus, this event probably constituted a significant fraction of the cross‐polar cap potential.

Data‐derived spatiotemporal resolution constraints for global auroral imagers

We present new data‐derived constraints on spatiotemporal resolution of global auroral imagers. The reported results are based on an extensive set of images from two previously flown instruments, POLAR UVI and IMAGE WIC, processed using the event detection methodology developed by Uritsky et al. (2002, 2003, 2006). We use the cross‐scale analysis of ground‐based and spacecraft observations of auroral emission regions by Kozelov et al. (2004) to derive the power law exponent relating spatial and temporal scales of auroral precipitation events, and estimate the normalization factor entering this relation using the satellite data. Our results show the existence of a nontrivial scaling relation between the relaxation time and the spatial dimension of auroral emission events. We use this relation as a proxi to the resolution scaling function providing non‐redundant combinations of spatial and temporal resolution of an optimized auroral imager consistent with the dynamics of multiscale auroral precipitation.

Substorm cycle dependence of various types of aurora

We combine 4861 identifications of substorm onsets from satellite global imagers (Polar UVI and IMAGE FUV) with DMSP particle data to investigate the substorm cycle dependence of the diffuse aurora (electrons and ions), monoenergetic aurora (produced by quasi‐static electric fields), and broadband aurora (associated with Alfvénic acceleration). Although all types of aurora increase at substorm onset, broadband aurora shows a particular association with substorms and, especially, substorm onset. While diffuse electron and monoenergetic auroral precipitating power rises by 79% and 90%, respectively, following an onset, wave aurora rises by 182%. In the first 10–15 min following onset, the power associated with Alfvénic acceleration is comparable to monoenergetic acceleration (also called “inverted V” events). In general, this is not the case before onset, or indeed, during recovery. The rise time of the electron diffuse aurora following onset is much slower, about 50 min, and thus presumably extends into recovery. Discrete acceleration, which rises over just a few minutes, is already deep into decline, while diffuse auroral power is still rising. Most of the sharp increase in Alfvénic precipitating power at onset is associated with a 132% jump in the characteristic energy associated with broadband acceleration. Previous work has associated PiB signatures (which are compressional) in the magnetotail with dipolarization. More recently, FAST observations near the auroral zone have been used to show that there are nearly simultaneous sharp increases in Alfvén waves, apparently created by mode coupling. The present work strongly supports the idea that the Alfvénic aurora has a particular connection to substorm onsets, with much of the auroral power over a substorm cycle concentrated shortly after onset. Finally, a superposed epoch analysis of commonly used coupling functions indicates a drop in the mean solar wind driving starting 20 min before substorm onset. However, the distribution of increases and decreases in solar wind driving is roughly symmetric about a peak at zero (no change) but with (rare) large drops in solar wind driving (“northward turnings”) outweighing large increases. Comparison with the distribution of random changes in solar wind driving leads us to suggest that probably only a minority of substorms are externally driven.

Polar UVI and THEMIS GMAG observations of the ionospheric response to a hot flow anomaly

We present observations of the ionospheric response to a hot flow anomaly (HFA) interacting with the magnetosphere. On 4 July 2007, the THEMIS spacecraft observed an HFA, a disruption of the solar wind flow, on both sides of the bow shock. The ionospheric response was measured by global auroral images taken over the southern hemisphere by Polar UVI, by ground based magnetometers and photometers in Antarctica, and by the THEMIS Ground Based Observatory (GBO) magnetometers (GMAGs) in the northern hemisphere. Polar UVI observations show a region of enhanced auroral emission in the morning sector about 10 min after the THEMIS spacecraft observed the HFA. This region of enhanced emission was located above South Pole station which observed magnetic perturbations and an increase in the auroral luminosity. The THEMIS GMAGs in the conjugate hemisphere also observed magnetic perturbations consistent with a traveling convection vortex (TCV). Polar UVI tracked the spatial and temporal development of the region of enhanced emission. Slow anti-sunward motion was observed as the emission weakened and then re-brightened over the course of about 10 min. Simultaneously, the THEMIS GMAG array observed anti-sunward motion of the magnetic impulse with a velocity much greater than that of the auroral emission. These are the first simultaneous observations of localized enhanced auroral emission and the associated magnetic impulse propagating anti-sunward over a distance of several 1000 km. We suggest that the origin of the magnetic perturbation and resulting auroral emission is the deformation of the magnetopause due to the HFA–magnetosphere interaction. The different propagation speeds of the auroral emission in the southern hemisphere and the magnetic perturbation in the northern hemisphere imply either (1) a decoupling of the auroral emission and field-aligned current signature or (2) a decoupling of these processes between the two hemispheres. The large difference in the ionospheric conductivity between the northern (summer) and southern (winter) hemispheres may be an important factor in this decoupling. More observations are needed to adequately address the decoupling mechanism.

Relatively low‐latitude wave aurora and substorms

Wave aurora produces broadband particle acceleration. For many years, this type of signature was anecdotally but widely thought to occur primarily at the poleward edge of the auroral oval. Recent statistical work has found that, on the contrary, for strong solar wind driving, broadband aurora is common throughout the auroral oval. Of particular interest is the occurrence of wave aurora below 68 degrees MLAT. We have studied the dependence of such signatures, both as a function of solar wind driving, and as a function of substorm cycle. A set of about 1800 substorm onsets identified by Polar UVI, along with 116 isolated substorm onsets, were used for superposed epoch analysis. The results show that the wave aurora has a very strong substorm cycle dependence, rising dramatically in the first few minutes after onset, and peaking about 10–15 minutes later. By contrast, wave aurora has no easily identifiable solar wind driving peak, although the general trend is for wave aurora to become more common with higher driving (as do substorms themselves). This evidence adds to previous circumstantial evidence associating wave aurora particularly with substorms (for example, wave auroral power is much more strongly peaked in the premidnight region than is monoenergetic or “inverted‐V” aurora).

Azimuthal expansion of the dipolarization at geosynchronous orbits associated with substorms

Abstract. Dipolarization, one of the main indicators of substorm expansion onset, represents topological changes in the magnetotail. It is believed that dipolarization is initiated at a longitudinally confined region in the tail, followed by the azimuthal expansion. There are very limited studies in the literature regarding the azimuthal propagation of the dipolarization front in the magnetotail. In this study we have used ten years of GOES data and POLAR and IMAGE data to study the characteristics of the propagation of the dipolarization fronts at the geosynchronous orbit. We have identified a number of dipolarization events in the GOES magnetic field data and substorm onsets from POLAR UVI and IMAGE-FUV measurements. Using the delay of dipolarization signatures at the two GOES satellites and onset times from POLAR and IMAGE measurements we have estimated the propagation speed of the dipolarization fronts. The calculated speeds vary between 10 km/s and 420 km/s and show a power law distribution.

Collective dynamics of bursty particle precipitation initiating in the inner and outer plasma sheet

Abstract. Using multiscale spatiotemporal analysis of bursty precipitation events in the nighttime aurora as seen by the POLAR UVI instrument, we report a set of new statistical signatures of high- and low-latitude auroral activity, signaling a strongly non-uniform distribution of dissipation mechanism in the plasma sheet. We show that small-scale electron emission events that initiate in the equatorward portion of the nighttime auroral oval (scaling mode A1) have systematically steeper power-law slopes of energy, power, area, and lifetime probability distributions compared to the events that initiate at higher latitudes (mode B). The low-latitude group of events also contain a small but energetically important subpopulation of substorm-scale disturbances (mode A2) described by anomalously low distribution exponents characteristic of barely stable thermodynamic systems that are prone to large-scale sporadic reorganization. The high latitude events (mode

A state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail obtained from superposed epoch analysis

We have obtained a state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2‐min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north‐south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X ∼ −16 to −20 RE at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross‐tail current, which extends from X ∼ −5 to −20 RE in the premidnight sector. Then the plasmoid substantially evolves tailward of X ∼ −20 RE immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X ∼ −7 to −10 RE 2 min before onset. The dipolarization region then expands tailward as well as in the dawn‐dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high‐energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.

Scale‐free and scale‐dependent modes of energy release dynamics in the nighttime magnetosphere

Based on a spatiotemporal analysis of POLAR UVI images, we show for the first time that energy, power, area and lifetime probability distributions of electron precipitation events in the nighttime auroral oval have a significant latitudinal dependence. The low‐latitude group of the events contains a distinct subpopulation of strong auroral disturbances violating the uniform power‐law behavior reported in previous publications, while the high latitude group is described by nearly perfect power‐law statistics over the entire range of scales studied, in agreement with earlier findings. The results obtained indicate that the inner and outer portions of the plasma sheet may be characterized by substantially different scaling regimes of bursty energy dissipation suggestive of different universality classes and/or driving conditions associated with multiscale turbulence in these regions.

Pairs of solar wind‐magnetosphere coupling functions: Combining a merging term with a viscous term works best

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.

Aurora and open magnetic flux during isolated substorms, sawteeth, and SMC events

Abstract. Using Polar UVI LBHl and IMAGE FUV WIC data, we have compared the auroral signatures and polar cap open flux for isolated substorms, sawteeth oscillations, and steady magnetospheric convection (SMC) events. First, a case study of each event type is performed, comparing auroral signatures and open magnetic fluxes to one another. The latitude location of the auroral oval is similar during isolated substorms and SMC events. The auroral intensity during SMC events is similar to that observed during the expansion phase of an isolated substorm. Examination of an individual sawtooth shows that the auroral intensity is much greater than the SMC or isolated substorm events and the auroral oval is displaced equatorward making a larger polar cap. The temporal variations observed during the individual sawtooth are similar to that observed during the isolated substorm, and while the change in polar cap flux measured during the sawtooth is larger, the percent change in flux is similar to that measured during the isolated substorm. These results are confirmed by a statistical analysis of events within these three classes. The results show that the auroral oval measured during individual sawteeth contains a polar cap with, on average, 150% more magnetic flux than the oval measured during isolated substorms or during SMC events. However, both isolated substorms and sawteeth show a 30% decrease in polar cap magnetic flux during the dipolarization (expansion) phase.

Particle and field characteristics of broadband electrons observed by the FAST satellite during a geomagnetic storm

Broadband electrons (BBEs) are remarkable flux enhancements (&gt;1013 eV cm−2 s−1) of precipitating electrons over a broad energy range (0.03–30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. We show characteristics of particles (energy spectra and pitch angle distribution) and fields (electric field, magnetic field, and wave spectra) during a BBE event observed by the Fast Auroral SnapshoT (FAST) satellite. The BBEs were observed at an altitude of ∼2000 km at 59°–61° invariant latitudes (ILATs) and 21 h magnetic local time (MLT). The event was observed at ∼7 min after the onset of a substorm during the main phase of the Bastille Day geomagnetic storm (minimum Dst = −301 nT) on 15 July 2000. The precipitation region of the BBEs corresponded to a localized intensification of auroral emission, lasting ∼14 min, observed by the Polar UVI images at 50°–60° geomagnetic latitudes (MLATs) and 20–21 MLTs. These results suggest that rapid particle acceleration was occurring in the inner magnetosphere associated with a storm‐time substorm. The pitch angle distribution of BBEs was isotropic except for a loss cone feature around the field‐aligned upward direction at a higher energy range above ∼1 keV, while field‐aligned electron fluxes were larger than the perpendicular fluxes below ∼1 keV. These results imply that a higher energy part of the BBEs originated from higher altitudes in the inner magnetosphere and that a lower energy part was accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields were observed during this BBE event. From these results, we discuss possible acceleration of the lower energy part of BBEs through wave–particle interaction.