Showing papers on "Substorm published in 2000"

Penetration of auroral electric fields to the equator during a substorm

Abstract: We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low-latitude negative bay. This latitudinal profile implies that in addition to the three-dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field-aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66° corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijarvi (56° CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijarvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.

Evaluation of the tail current contribution to Dst

Abstract: The Dst index is produced using low-latitude ground magnetic field measurements and frequently is used as an estimate of the energy density of the ring current carried mainly by energetic (∼ 10 – 200 keV) ions relatively close to the Earth. However, other magnetospheric current systems can cause field perturbations at the Earth's surface: for example, dayside magnetopause currents are known to contribute to the Dst index. It has also been suggested that the nightside tail current sheet can significantly affect the Dst index during high magnetic activity periods when the currents are intense and flow relatively close to the Earth. In this study, several disturbed periods are input into Tsyganenko magnetic field models. From the time series of the external and internal fields an artificial Dst index is computed using the same procedure followed in the actual Dst calculation. A tail region in the magnetosphere is explicitly defined and the T96 and T89 models are used to calculate the effect of current within this tail region on ground measurements and therefore on Dst. The results are then compared with the measured Dst to determine the tail current contribution to Dst. It is found that for a geomagnetic storm and a storm-time substorm with Dst of ∼ 80 nT the tail current contribution is between 22 and 26 nT. The same analysis is also applied to several isolated non-storm-time substorms, yielding a nearly linear relationship between Dst and the tail current contribution. This contribution is approximately one quarter of Dst.

High energy neutral atom (hena) imager for the IMAGE mission

Abstract: The IMAGE mission will be the first of its kind, designed to comprehensively image a variety of emissions from the Earth’s magnetosphere, with sufficient time resolution to follow the dynamics associated with the development of magnetospheric storms. Energetic neutral atoms (ENA) emitted from the ring current during storms are one of the key emissions that will be imaged. This paper describes the characteristics of the High Energy Neutral Atom imager, HENA. Using pixelated solid state detectors, imaging microchannel plates, electron optics, and time of flight electronics, HENA is designed to return images of the ENA emitting regions of the inner magnetosphere with 2 minute time resolution, at angular resolution of 8 degrees or better above the energy of ∼ 50 keV/nucleon. HENA will also image separately the emissions in hydrogen, helium, and oxygen above 30 keV/nucleon. HENA will reject energetic ions below 200 keV/charge, allowing ENA images to be returned in the presence of ambient energetic ions. HENA images will reveal the distribution and the evolution of energetic ion distributions as they are injected into the ring current during geomagnetic storms, as they drift about the Earth on both open and closed drift paths, and as they decay through charge exchange to pre-storm levels. Substorm ion injections will also be imaged, as will the regions of low altitude, high latitude ion precipitation into the upper atmosphere.

Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves

Abstract: On August 20, 1996, balloon-borne X-ray detectors observed an intense X-ray event as part of a French balloon campaign near Kiruna, Sweden, at 1532 UT (1835 magnetic local time), on an L shell of 5.8. The energy spectrum of this event shows the presence of X rays with energies > 1 MeV, which are best accounted for by atmospheric bremsstrahlung from monoenergetic ∼1.7 MeV precipitating electrons. Ultraviolet images from the Polar satellite and energetic particle data from the Los Alamos geosynchronous satellites show the onset of a small magnetospheric substorm 24 min before the start of the relativistic electron precipitation event. Since the balloon was south of the auroral oval and there was no associated increase in relativistic electron flux at geosynchronous altitude, the event is interpreted as the result of selective precipitation of ambient relativistic electrons from the radiation belts. Pitch angle scattering caused by resonance with electromagnetic ion cyclotron mode waves is the most likely mechanism for selective precipitation of MeV electrons. A model is presented in which wave growth is driven by temperature anisotropies in the drifting substorm-injected proton population. The model predicts that this wave growth and resonance with ∼1.7 MeV electrons will occur preferentially in regions of density >10 cm−3, such as inside the duskside plasmapause bulge or detached plasma regions. The model predictions are consistent with the location of the balloon, the observed energies, and the timing with respect to the substorm energetic particle injection.

Electromagnetics of substorm onsets in the near‐geosynchronous plasma sheet

Abstract: A search of the CRRES database identified 20 events in which the satellite was located within the local-time sector spanned by the substorm current wedge (SCW) as it formed. Poynting vectors for low-frequency waves are derived from the electric and magnetic field measurements. In 19 of the events, data are inconsistent with the notion that the SCW initiates from the braking of earthward bulk flows emanating from a near-Earth X line. Rather, the data support drift-Alfven ballooning in the near-geosynchronous plasma sheet as being responsible for initiation of the SCW and substorm onset. Dipolarization at CRRES is preceded by eastward excursions of the electric field (trigger waves), at which time the first significant electromagnetic energy is observed flowing toward the ionosphere. Dipolarization and the SCW appear before ground onset, following one or more of these trigger waves. The so-called “explosive growth phase” occurs in association with explosive growth of the trigger waves soon after onset. Seven characteristic features of substorm onsets and expansions observed at CRRES are described. Among these are two stages of expansion. The first expansion stage is initiated by the trigger waves (ballooning) in the near-geosynchronous plasma sheet. Approximately 10 minutes later a second stage begins consistent with the arrival of earthward bulk flows emanating from a near-Earth X line. Near-geosynchronous substorm onsets can explain the observed increase in the occurrence rate of fast bulk flows earthward of its minimum value near X = −12 RE. Drift-Alfven ballooning also provides a possible causal link between observed reductions of the solar wind driver and substorm onsets.

Evaluation of low-latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery

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

The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere

Abstract: The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral-electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local, maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 less than or equal to L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3+/-1.2 and 4.6+/-0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave-particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 less than or equal to L < 6.0 the timescale for wave decay at the ECH equator is 2.3 +/- 0.6 and 1.1 +/- 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time.

MHD simulation of the magnetotail during the December 10, 1996, substorm

Abstract: This paper presents results of a global MHD simulation of a substorm that occurred on December 10, 1996. We concentrate on the relationship between the simulation results and the magnetotail observations during the growth and expansion phases of the substorm. In general, we find excellent agreement between the single point observations made by various spacecraft in both the geosynchronous and mid-tail regions: the simulation accurately represented the energy loading (lobe field increase), small-scale activations (partial dipolarizations), and a global substorm onset (large dipolarizations and fast flows). The global view presented by the simulation shows complex series of discrete flow channels during the expansion phase prior to the onset of global reconnection. It is these flows channels that disrupt the thin current sheets present during the expansion phase of the substorm.

Phase transition‐like behavior of the magnetosphere during substorms

Abstract: The behavior of substorms as sudden transitions of the magnetosphere is studied using the Bargatze et al. [1985] data set of the solar wind induced electric field vBs and the auroral electrojet index AL. The data set is divided into three subsets representing different levels of activity, and they are studied using the singular spectrum analysis. The points representing the evolution of the magnetosphere in the subspace of the eigenvectors corresponding to the three largest eigenvalues can be approximated by two-dimensional manifolds with a relative deviation of 10–20%. For the first two subsets corresponding to small and medium activity levels the manifolds have a pleated structure typical of the cusp catastrophe. The dynamics of the magnetosphere near these pleated structures resembles the hysteresis phenomenon typical of first-order phase transitions. The reconstructed manifold is similar to the “temperature-pressure-density” diagrams of equilibrium phase transitions. The singular spectra of vBs, AL, and combined data have the power law dependence typical of second-order phase transitions and self-organized criticality. The magnetosphere thus exhibits the signatures of both self-organization and self-organized criticality. It is concluded that the magnetospheric substorm is neither a pure catastrophe of the low-dimensional system nor a random set of avalanches of different scales described by the simple sandpile models. The substorms behave like nonequilibrium phase transitions, with features of both first- and second-order phase transitions.

Large Alfvén wave power in the plasma sheet boundary layer during the expansion phase of substorms

Abstract: Observations by the Polar satellite of large Poynting flux in the plasma sheet boundary layer at geocentric distances of 4 to 6 RE and between 22 and 3 hrs magnetic local time were correlated with H-bay signatures from ground magnetometer records. We provide evidence that large Poynting fluxes occur during the substorm expansion phase. The Poynting fluxes exceeded 1 ergs/cm²s (125 ergs/cm²s when mapped to 100 km), were dominantly directed toward the ionosphere, and were associated with Alfven waves. These observations demonstrate the importance of Alfven wave power as a means of energy transport from the distant magnetotail to the ionosphere during the most dynamic phase of substorms.

A statistical study of variations in the near and middistant magnetotail associated with substorm onsets: GEOTAIL observations

Abstract: We have studied the three-dimensional structure of substorm-associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn-dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn-dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near-Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin-current reconnection model.

The state transition model of the substorm onset

Abstract: The substorm mechanism is investigated from a resistive magnetohydrodynamic simulation under the assumption that the magnetotail becomes more diffusive as it goes further downtail. The simulation uses the finite volume total-variation diminishing scheme on an unstructured grid system to evaluate the magnetosphere-ionosphere coupling effect more precisely and to reduce the numerical viscosity in the near-Earth plasma sheet. The calculation started from a stationary solution under a northward interplanetary magnetic field (IMF) condition with non-zero IMF By. After a southward turning of the IMF the simulation results show the progress of plasma sheet thinning in the magnetosphere, together with increases in the size of the auroral oval and in the magnitude of the field aligned current (FAC) in the polar ionosphere. This thinning is promoted by the drain of closed flux from the plasma sheet occurring under the enhanced convection. In this stage the reclosure process in the plasma sheet which determines the flux piling up from the midtail to the near-Earth plasma sheet is not so effective, since it is still controlled by the remnant of northward IMF. The substorm onset occurs as an abrupt change of pressure distribution in the near-Earth plasma sheet and an intrusion of convection flow into the inner magnetosphere. After the onset the simulation results reproduce both the dipolarization in the near-Earth tail and the near-Earth neutral line (NENL) at the midtail, together with plasma injection into the inner magnetosphere and an enhancement of the nightside FAC. Dipolarization is hastened by a northward re-turning of the IMF, indicating that it is triggered off through the breakdown of dynamic stress balance in the near-tail plasma sheet established under the convection-controlling tail thinning. It is concluded that the direct cause of the onset is the dipolarization, which is not a mere pileup of the flux ejected from the NENL but the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organizing criticality. In this idea the NENL is a topologically controlled diffusive process and not directly responsible for the onset, although it constitutes an important part in the convection system necessary for the state transition. After the onset the simulation results show a thickening of the plasma sheet and a tailward retreating of the NENL. Under a continuously southward IMF this state with a thickened plasma sheet shows a signature of the steady magnetospheric convection event.

Particle transport and energization associated with substorms

Abstract: Energetic particle flux enhancement events observed by satellites during substorms are studied by considering the interaction of particles with earthward propagating electromagnetic pulses of westward electric field and consistent magnetic field of localized radial and azimuthal extent in a background magnetic field. The energetic particle flux enhancement is mainly due to the betatron acceleration process: particles are swept by the earthward propagating electric field pulses via the E×B drift toward the Earth to higher magnetic field locations and are energized because of magnetic moment conservation. The most energized particles are those which stay in the pulse for the longest time and are swept the longest radial distance toward the Earth. Assuming a constant propagating velocity of the pulse, we obtain analytical solutions of particle orbits. We closely examine substorm energetic particle injection by computing the particle flux and comparing with geosynchronous satellite observations. Our results show that for pulse parameters leading to consistency with observed flux values, the bulk of the injected particles arrive from distances of less than 9 RE, which is closer to the Earth than the values obtained by the previous model (Li et al., 1998).

Acceleration of oxygen ions of ionospheric origin in the near-Earth magnetotail during substorms

Abstract: Measurements from the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionospheric origin at energies of 9 keV to 210 keV in the near-Earth plasma sheet during the substorm expansion phase. Substorm signatures were clearly observed on the ground at 1850 UT on May 17, 1995. In the expansion phase of this substorm, Geotail stayed in the plasma sheet at X∼−10.5 RE and observed a local dipolarization signature accompanied by strong disturbances of the magnetic field. From the energetic ion flux data of EPIC/STICS, we obtained the following results: (1) energetic flux enhancement was more pronounced for O+ than for H+; (2) the flux was enhanced almost simultaneously with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before substorm onset) relative to the H+ flux ratio, was as large as 1.31; and (4) thermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 keV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 events of local dipolarization found in the near-Earth region (X∼−6 to −16 RE). We found that EO+ is larger than unity in all ranges of radial distance and that the average value of EO+ is 1.37. These results suggest that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a mechanism in which ions are accelerated in a non-adiabatic way during substorm-associated field reconfiguration.

Loading-unloading processes in the nightside ionosphere

Abstract: The loading-unloading processes during magnetospheric substorms are examined using the ’IL index’ from the IMAGE magnetometer chain together with solar wind and IMF data from the WIND spacecraft. The IL index is a local, midnight sector, AL index. Energy input throughout the substorm is evaluated by integrating the epsilon parameter from the start of the growth phase until the end of the substorm. Energy dissipated in the ionosphere is estimated by integrating the IL index from the substorm onset to the end of the substorm. We show that the best correlation with the energy dissipated in the ionosphere is given by the energy input to the system after the substorm onset. Hence, we conclude that the energy loaded during the growth phase is necessary for the magnetospheric reconfiguration before the substorm onset, but that the size of the substorm as measured by the IL index is mostly governed by the direct energy input during the expansion phase.

How northward turnings of the IMF can lead to substorm expansion onsets

Abstract: The frequent triggering of the expansion phase of substorms by northward turnings of the interplanetary magnetic field (IMF) can be understood in terms of the existence of two neutral points. The distant neutral point produces a plasma sheet on closed field lines that resupplies the magnetized plasma surrounding the near-Earth neutral point. As long as the near-Earth neutral point reconnects in moderately dense plasma, the reconnection rate is low. When the IMF turns northward, reconnection at the distant neutral point ceases but reconnection at the near-Earth neutral point continues and soon reaches open, low density magnetic field lines where the rate of reconnection is rapid, and a full expansion phase occurs. This model is consistent with the observations of substorms with two onsets: an initial one at low invariant latitudes when reconnection at the near Earth neutral point first begins and the second when reconnection reaches low density field lines at the edge of the plasma sheet and continues into the open flux of the tail lobes. It is also consistent with the occurrence of pseudo breakups in which reconnection at the near Earth neutral point begins but does not proceed to lobe field lines and a full expansion phase.

The impact of geomagnetic substorms on GPS receiver performance

Abstract: During the current period of solar maximum, there is concern within the GPS community regarding GPS receiver performance during periods of intense geomagnetic substorms. Such storms are common in the high latitude auroral region, and are associated with small-scale scintillation effects, which can cause receiver tracking errors and loss of phase lock. The auroral oval can extend many degrees equatorward under active ionospheric conditions, with an impact on precise positioning applications in Canada, the United States and Northern Europe. In this paper, a study of receiver tracking performance is conducted during periods of auroral substorm activity. Dual frequency observations are obtained using codeless and semicodeless GPS receivers (Trimble 4000SSi, NovAtel MiLLennium and Ashtech Z-12), and performance comparisons are established and interpreted with respect to GPS availability at solar maximum and the years beyond.

The nightside poleward boundary of the auroral oval as seen by DMSP and the Ultraviolet Imager

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

Can substorms produce relativistic outer belt electrons

Abstract: In an effort to explain how magnetic storms can cause strong enhancements in outer belt MeV electrons, we have studied the substorm-associated acceleration of energetic (tens of keV) plasma sheet electrons and their injection into the outer-trapped region of the magnetosphere. The study is based on tracing test particles in three-dimensional MHD simulations of substorm dipolarization. The simulation-based electric and magnetic fields [Birn and Hesse, 1996] exhibit a strong earthward collapse around local midnight, leading to magnetic field dipolarization along with a corresponding induction electric field. The test particle traces show that tens of keV plasma sheet electrons can be transported from about x ≈ −20 RE to x ≈ −10 RE and can gain about a factor of 10 in energy. If these particles are further transported inward to L ∼ 6 while conserving the first adiabatic invariant, they will have energies of an MeV or more. In the substorm acceleration process the dominant energy gain occurs during the earthward radial transport by E × B drift in the strong dipolarization region. In our calculations the electrons that gain the most energy are ones that circle a local maximum in the equatorial magnetic field strength and thus spend a relatively long time in the region of collapsing field. The first adiabatic invariant is broken at the beginning of the particle trajectory, near the neutral line, where the magnetic curvature radius can be comparable to the particle gyroradius. The second adiabatic invariant is also broken later in the process, when particles can be temporarily trapped in off-equatorial magnetic minima associated with MHD waves on the dipolarizing field lines. Estimation of the number of accelerated plasma sheet electrons indicates that the Birn-Hesse substorm, which is not particularly large, produces only ∼ 2% of the number of MeV electrons observed in a typical post storm outer belt electron enhancement. A series of substorms, some of them large, might produce a large enough enhancement, but it should be noted that this is an order of magnitude estimate, and it is rather sensitive to plasma sheet and substorm parameters.

Entry of plasma sheet particles into the inner magnetosphere as observed by Polar/CAMMICE

Abstract: Statistical results are presented from Polar/CAMMICE measurements of events during which the plasma sheet ions have penetrated deeply into the inner magnetosphere. Owing to their characteristic structure in energy-time spectrograms, these events are called “intense nose events.” Almost 400 observations of such structures were made during 1997. Intense nose events are shown to be more frequent in the dusk than in the dawn sector. They typically penetrate well inside L = 4, the deepest penetration having occurred around midnight and noon. The intense nose events are associated with magnetic (substorm) activity. However, even moderate activity (AE = 150–250 nT) resulted in formation of these structures. In a case study of November 3, 1997, three sequential inner magnetosphere crossings of the Polar and Interball Auroral spacecraft are shown, each of which exhibited signatures of intense nose-like structures. Using the innermost boundary determinations from these observations, it is demonstrated that a large-scale convective electric field alone cannot account for the inward motion of the structure. It is suggested that the intense nose structures are caused by short-lived intense electric fields (in excess of ∼1 mV/m) in the inner tail at L=4–5.

Auroral disturbances during the January 10, 1997 magnetic storm

Abstract: It is well known that intense and frequent auroral-zone disturbances, often attributed to substorms, occur during magnetic storms. We examine observations during the January 10, 1997 main phase and find that observed auroral-zone activity was dominated by a combination of global auroral and current enhancements, which are a direct response to solar wind dynamic pressure enhancements, and poleward boundary intensifications, which are localized in longitude and have an auroral signature that moves equatorward from the magnetic separatrix. Poleward and azimuthally expanding regions of auroral activity which accompany substorms are found to contribute significantly less to the observed activity. This suggests that poleward boundary intensifications and dynamic pressure responses may be an important cause of disturbances during periods of enhanced convection such as magnetic storms and convection bays.

Findings concerning the positions of substorm onsets with auroral images from the Polar spacecraft

Abstract: High-resolution global images of Earth's auroras in the visible emissions of atomic oxygen at 557.7 nm with a camera on board the Polar spacecraft were used to determine the magnetic latitude of the auroral arc which brightens at the onset of a substorm. Six events were analyzed: an intensification and an onset on December 17, 1997, three substorms on January 6, 1998, and a substorm on March 1, 1999. The Tsyganenko [1989] model for the global magnetic field was used to map the field lines threading the center of the auroral arc to equatorial distances. During the period of relative magnetic quiescence on December 17 the equatorial distances were ∼9 RE. During the magnetically disturbed periods of January 6 and March 1 the magnetic field lines for the auroral onset brightenings were mapped to equatorial distances in the range of 5–7 RE, and thus in the region of the extraterrestrial ring current. The equatorial position of the field lines was independently verified for the substorm on March 1 by observing the atmospheric footprint of proton precipitation from the ring current. This footprint was seen in the emissions of atomic hydrogen at 656.3 nm. The onset arc was located near the earthward edge of the ring current at magnetic shell parameter L = 4 RE and within 1 RE of the equatorial position computed with the global magnetic field model. The equatorial distance of the elusive substorm onset is important in establishing plasma parameters for quantitative evaluation of candidate mechanisms for the explosive instability that initiates substorms.

Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations

Abstract: Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

Small substorms: Solar wind input and magnetotail dynamics

Abstract: We investigated properties of 43 small magnetospheric substorms Their general signatures were found to be consistent with the so-called contracted oval or northern Bz substorms Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading-unloading scheme as the larger ones However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF By and small fluctuating IMF Bz Plasma sheet signatures could be very strong and likely were localized in their cross-tail size Negative bays in auroral X magnetograms were of order of 100–300 nT, with maxima at Bear Island station (71°geomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes Small substorms probably differ from their larger counterparts in a way that coherency of the magnetotail reconfiguration in the inner and middle-tail regions and across the tail is lost in smaller substorms

Auroral kilometric radiation at substorm onset

Abstract: The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ±1 (±2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm-associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ∼1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ∼300 kHz, corresponding to an upper limit of the AKR source altitude of ∼4700 km. The entire AKR frequency band expands to ∼64–650 kHz within a few minutes after onset, corresponding to a wider source altitude of ∼2100–12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase.

Current‐driven electromagnetic ion cyclotron instability at substorm onset

Abstract: ULF waves at frequencies of the order of the proton gyrofrequency are systematically detected at the early development of substorm breakups. The observed characteristics of these ULF waves, namely their polarization and δE/δB ratio are consistent with being electromagnetic waves driven unstable by a parallel current. In order to take into account properly wave particle interactions, a kinetic approach is used. We show that a parallel drift between electrons and ions leads to a strong instability, resulting from a coupling between the shear Alfven (SA) mode and the fast magnetosonic mode via this drift. We call it current‐driven Alfven instability (CDA). We have carried out a parametric study of this current‐driven electromagnetic instability in a parameter range adapted to conditions prevailing at the geostationary orbit before and during breakup. We conclude that even a modest parallel drift between electrons and ions (Vd), caused by a parallel current, can destabilize CDA waves. When the ratio between Vd/VA (VA being the Alfven velocity) increases, the CDA mode couples with SA mode. These two modes have a substantial parallel electric field that leads to a fast parallel diffusion of the electrons. We suggest that this parallel diffusion leads to an interruption of the parallel current.

Height-integrated conductivity in auroral substorms. 1. Data

Abstract: We present height-integrated Hall and Pedersen conductivity (conductance) calculations from 31 individual Dynamics Explorer 2 (DE 2) substorm crossings. All are northern hemisphere (except one) nighttime passes which took place from September 1981 to January 1982. Global auroral images are used to select substorms which display a typical bulge-type auroral emission pattern and to organize the position of individual DE 2 passes with respect to key features in the emission pattern. The Hall and Pedersen conductances are calculated from electron precipitation data obtained by the low altitude plasma instrument (LAPI) carried on DE 2 and the monoenergetic conductance model by Reiff [1984]. This method is shown to effectively minimize undesirable smearing of parameters in statistical substorm studies. Large spatial gradients in the conductance profiles are common in high-latitude part of the premidnight substorm region. The conductances maximizes in the high-latitude part of the surge with average Hall and Pedersen conductances of 38 and 18 mho respectively. During six different DE 2 passes we found Hall conductance peaks exceeding 100 mho in the high-latitude part of the surge or surge horn. These peaks are highly localized with a typical scale size of ∼20 km and are associated with energetic (>10 keV) inverted V events. Except in the low-latitude part of the auroral oval the Hall to Pedersen ratio equals or exceeds 1.0, and it peaks in the high-latitude part of the surge where values of 3 or more are common. The latitudinal conductance profiles are strongly asymmetric and have a pronounced local time dependency.

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

Abstract: On August 21st 1998, a sharp southward turning of the IMF, following on from a 20 h period of northward directed magnetic field, resulted in an isolated substorm over northern Scandinavia and Svalbard. A combination of high time resolution and large spatial scale measurements from an array of coherent scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive substorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement of the electron temperature and UV aurora. This poleward expanding auroral structure moves initially at 0.9 km s-1 poleward, finally reaching a latitude of 72.5°. The optical signature expands rapidly westwards at ~6 km s-1, whilst the eastward edge also expands eastward at ~0.6 km s-1. Typical flows of 600 m s-1 and conductances of 2 S were measured before the auroral activation, which rapidly changed to ~100 m s-1 and 10–20 S respectively at activation. The initial flow response to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, imposed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of low-electric field, but also low conductivity poleward of the high-conductivity region. Rapid flows are observed at the edge of the high-conductivity region, and subsequently the high flow region develops, flowing around the expanding auroral feature in a direction determined by the flow pattern prevailing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2° further poleward than the UV auroral signature associated with it.