Phase Of Substorm Research Articles

A model for the propagation of the westward traveling surge

A unified model is developed for the propagation of the westward traveling surge (WTS) that can explain the diversity in the observed surge characteristics. We start with the Inhester‐Baumjohann model for the surge region, which implicitly includes both the Hall and Pedersen currents. It is found that precipitating electrons at the conductivity gradient modify the gradient, causing it to propagate as a wave front. The velocity of propagation is directly dependent on the ionization efficiency of the precipitating electrons and therefore increases dramatically when they become more energetic during substorm onsets. For example, we predict that when the incident electron energy changes from 1 keV to 10 keV the surge velocity should increase from 2 km/s to 34 km/s. The direction of the surge motion depends on the presence of polarization charges on the poleward surge boundary. This is related to the efficiency with which the poleward ionospheric currents are closed off into the magnetosphere by the field‐aligned currents. Inclusion of the electron‐ion recombination rate modifies the surge propagation velocity and leads to explicit expressions for the conductivity profile. Sufficient precipitation current is required to overcome electron‐ion recombination in order for the surge to expand. When the precipitating current is less than this threshold the WTS retreats. Therefore, the model describes the ionospheric response to both the expansion and recovery phases of the magnetic substorm.

Fast variations of ground irradiance in auroral regions

Variations in ground irradiance at frequencies higher than 0.1 Hz may severely degrade the performance of automatic imaging systems. For this reason measurements and analysis of fast variations in ground irradiance in the auroral region due to variable auroral emissions have been performed. The largest amplitudes and fastest variations are found to occur during the active phase of geomagnetic substorms and during pulsating auroral conditions, respectively. The power spectra of the variations show a rapid decrease toward higher frequencies. Pulsating auroral conditions contain more power at higher frequencies than do the active phase. On the average ~10% of the total power in the spectra is at frequencies higher than 0.3 Hz.

Coordinated measurements of low‐energy electron precipitation and scintillations/TEC in the auroral oval

A case study of coordinated observations of low‐energy (<500 eV) electron precipitation in the auroral oval from DMSP/F2 and phase and amplitude scintillations from Goose Bay, using a geostationary satellite transmitting at 244 MHz, is presented. The precipitation event took place during the expansion phase of an intense evening substorm, when the equatorward boundary of the diffuse aurora reached 59° invariant latitude. Particularly large phase scintillations (>10 radians for fluctuation frequencies >0.0067 Hz) were found to be well correlated with intense fluxes (>109 particles (cm2 s sr)−1) of precipitated low‐energy electrons. Total electron content and magnetometer measurements indicate that the onset of the precipitation event was about 10 min prior to the DMSP pass. Within this time scale, the ionization generated in the F region could reach the topside so that the thermal sensor on board the DMSP satellite was able to measure a factor of 2–3 density enhancement at 840 km. The latitudinal width of these density structures is consistent with that of F region blobs observed at Chatanika. The gradient scale length measured in the topside was only 30 km, which was probably responsible for the fast growth rate of the scintillation‐producing irregularities. The phase to amplitude scintillation ratio changed rather drastically compared to quiet magnetic times, however, implying that increased convection velocities during these magnetic disturbances were partially responsible for the enhanced phase scintillation.

Magnetospheric processes preceding the onset of an isolated substorm: A Case study of the March 31, 1978, substorm

We examined in detail the effect of a southward turning of the interplanetary magnetic field (IMF) on the state of the magnetosphere, taking advantage of the availability of the data from IMS magnetometer meridian chains and from several spacecraft. A clear onset substorm occurred on March 31, 1978, when the magnetometer stations were located in the midnight to morning sector and the spacecraft were near the equatorial plane of the nightside magnetosphere. The onset time of the substorm expansion phase could be determined unambiguously in terms of both ground‐based magnetic and auroral signatures, and there was an interval lasting about 1 hour between the IMF southward turning and this onset. In this intervening interval the ionospheric current system of the DP 2 type developed. This enhancement of the ionospheric current was driven directly by the solar wind‐magnetosphere coupling. The onset of the expansion phase was then associated with the decrease in the magnetic field energy density in the tail, providing evidence that the substorm energy was supplied by the release (unloading) of energy from the tail. It is most likely that substorm energy dissipated in the auroral ionosphere throughout this relatively isolated and simple event was supplied by two components, ‘directly driven’ and ‘loading‐unloading’, the relative importance of which varied depending on the different substorm phases.

Comparison of phase and amplitude structures of Pc 5 pulsations in the morning and afternoon sectors

Spatial phase and amplitude structures of the morning and afternoon Pc 5 pulsations are compared using the North American IMS magnetometer network data. Comparisons of phases and amplitudes between different stations were made by a cross spectral analysis. The magnetic local time (MLT) of the events analyzed extends from 0500 to 1100 for the morning and from 1200 to 2100 for the afternoon. All events were observed either during quiet periods or during the recovery phases of substorms. The following results are obtained: (1) The spatial phase structures are very similar in the morning and afternoon sectors for each of the three components. (2) The amplitudes are larger in X than in Y for both sectors, where X and Y are the magnetic north and east components, respectively. (3) The latitudinal variations of the amplitude and of the phase differences between X and Y show dissimilarities in the morning and afternoon sectors. These results suggest that the magnetic field oscillations in the compressional mode, which are believed to dominate in the afternoon magnetosphere, are not observed on the ground in this time sector. As seen from the ground observations, the pulsations in both time sectors are caused by oscillations of the magnetospheric field in the transverse mode. The theory of the resonant oscillation of the magnetic field lines, which successfully explains the pulsations in the morning sector, does not seem to be applicable to the pulsations in the afternoon sector.

A two‐dimensional simulation of the interaction of the plasma sheet with the lobes of the Earth's magnetotail

A code involving two spatial dimensions and three velocity dimensions is used to model the tail of earth's magnetosphere in a continuation of earlier work using a one‐dimensional code. The model assumes the existence of an externally imposed convection electric field and the presence of cold particles in the lobes of the magnetotail. The model reproduces the results of the previous one‐dimensional model. In addition, the two‐dimensional model exhibits the development of turbulence which persists so long as cold lobe particles are being convected into the neutral sheet. The model also exhibits the earthward jetting of plasma, which becomes particularly intense after the tail field begins to collapse toward a more dipolar configuration. Magnetic energy is converted into energy of bulk motion. The model is used to identify the physical processes that may occur during the growth and recovery phases of substorms. It is also shown that if the convection electric field is strong enough, the magnetic energy of the tail will increase, resulting in a more extended magnetotail.

Electrojet boundaries and electron injection boundaries.

From observations of the magnetic field along several magnetometer chains motions of the boundaries of auroral electrojets during the active substorm phase have been derived. It turned out that for both electrojets (westward and eastward electrojet) the equatorward boundary is more stable than the poleward one. For both electrojets, however, this boundary correlates quite well with Kp. The Kp dependence of the equatorward electrojet boundary was compared with that of the electron injection boundary after KIVELSON et al. (1980). It turned out that the equatorward boundary of the westward electrojet—but not of the eastward electrojet—coincides quite well with that of the electron injection boundary.

Neutral line motion due to reconnection in two-ribbon solar flares and magnetospheric substorms

Two kinematic models of line-tied reconnection are considered which describe the motion of a magnetic neutral line (NL) during the main phase of a two-ribbon solar flare and during the recovery phase of a magnetospheric substorm in the geomagnetic tail. The models are kinematic in that they use only the magnetic induction equation, which suffices to determine the position and velocity of the NL as functions of time if the rate of reconnection is prescribed. The solar flare model shows that the observed large decrease in the rate at which “post”-flare loops rise upward from the photosphere during the main phase does not require a corresponding decrease in the rate of reconnection. Instead it is found that a constant rate of reconnection can account for the motion of the loops for almost the entire period during which they are observed. By contrast, application of the same procedures to the recovery phase of the magnetospheric substorm in the tail predicts a slightly increasing speed of NL motion if the rate of reconnection is constant. Furthermore, it is found that the motion of the NL relative to the ambient medium may account for much of the observed asymmetry in the magnetic field in the plasma sheet during recovery. Due to this motion, the plasma sheet thickness may be up to 4 times smaller and the normal magnetic field component up to 2 times weaker in the region tailward of the NL than in the corresponding region earthward of the NL.

The two‐component auroral electrojet

By using the AL index for one year as well as some of the IMS meridian chain records, the longitudinal center of the substorm westward electrojet is examined. Although on the average AL is most often dominated by stations located near 0300 MLT, the westward electrojet during the expansion and maximum phase of substorms tends to be centered in the premidnight to early morning sector, and, during the recovery phase, it is most intense in morning hours. It is suggested that the substorm‐associated westward electrojet consists of two components which have different characteristic times; one produced mainly by Hall conductivity enhancements which dominate in the midnight sector and the other one by southward electric fields in the morning sector.

Observations of inverted-V electron precipitation

The characteristics of inverted-V electron precipitation fluxes deduced predominantly from observations by the Atmosphere Explorer satellites are reviewed. The energy and pitch angle distributions are presented and shown to be generally in agreement with acceleration by a parallel electrostatic potential. Characteristics of secondary electrons are examined, and effects of beam plasma instabilities on these electrons are discussed. The properties of the monoenergetic component are compared with theoretical models of creating parallel DC electric fields, and found to favor the anomalous resistivity model. The article also discusses relations of inverted-V events with other auroral phenomena including auroras, electrostatic shocks, convective electric field reversals, field-aligned currents and wave emissions. The principal conclusions are: (1) plasma sheet electrons are continuously accelerated to form inverted-V structures in the pre-midnight hemisphere independent of substorm phase, (2) the acceleration processes are probably related to large scale electrostatic wave turbulence observed at altitudes of a few thousand kilometers, (3) narrow bursts of intense electron precipitation fluxes are found to be imbedded within some inverted-V's. It is argued that the narrow bursts of intense electron precipitation have the proper characteristics to cause discrete auroral arcs in the atmosphere. We suggest that these narrow bursts are accelerated by an electrostatic shock at higher altitude and capable of producing discrete auroral arcs below the observing satellite.

Three‐dimensional computer modeling of dynamic reconnection in the geomagnetic tail

The unstable dynamic evolution of the magnetotail in response to a sudden occurrence or enhancement of resistivity is studied by means of a three‐dimensional, time‐dependent, nonlinear, resistive MHD code. We start from a realistic three‐dimensional self‐consistent tail model, including flaring in y and z and plasma sheet thickening toward the flanks. Many features that are believed to characterize the expansion phase of substorms developed without any driving mechanism at the boundary. Confirming earlier two‐dimensional results, we found plasma sheet thinning, occurrence of strong flow and negative Bz, induced electric fields and formation of neutral lines, and a tailward‐moving ‘magnetic bubble’ or ‘plasmoid.’ In addition, the results show that the occurrence of Bz <0 and the shape of the neutral lines are more complex than in 2‐D models, with the negative Bz generally more restricted in time and space than strong tailward flow. The increase in plasma sheet thickness and Bz toward the flanks of the tail causes the reconnection and acceleration process to be limited in the y direction. Earthward from the main x line, a diversion of the cross‐tail current is found, being earthward on the dawn side of the tail and tailward on the dusk side. Although the magnetic field turns so as to stay nearly perpendicular to the diverted current, net field‐aligned components of the current remain inside the plasma sheet. These have the signatures of region 2 currents (Iijima and Potemra, 1976) flowing tailward on the dawn side and earthward on the dusk side.

Equatorward shift of the cusp during magnetospheric substorms

It is shown that the magnetic field of an enhanced dynamo current in the dayside boundary layer and of the connected circuit can quantitatively account for the equatorward shift of the cusp region which is observed during the expansive phase of magnetospheric substorms.

Evidence for the tailward retreat of a magnetic neutral line in the magnetotail during substorm recovery

On several occasions during the recovery phase of substorms the Los Alamos/MPI analyzers on the ISEE 1 and 2 satellites have observed protons counter‐streaming along the magnetic field within about 1 Re of the northern (upper) surface of the plasma sheet. We present data for one of these periods (04:00‐04:30, March 1, 1978) which strongly suggest that the counter‐streaming of the protons results from the mirroring of particles moving Earthward along the magnetic field. Only the Earthward flow is observed when the satellites first encounter the plasma sheet. But about 2 minutes after the entry into the plasma sheet the return tail ward flow appears while the Earthward flow continues. The speeds of both the Earthward and tailward moving populations thereafter show a systematic decrease with time with the tailward speed being persistently ∼200 km/sec greater than the earthward speed. Analysis of the correlation between the speeds of the two populations shows that the decrease in speed results from the relative motion of the satellites with respect to a spatial gradient in the velocity at the boundary of the plasma sheet. We interpret the time delay between the onset of the Earthward and tailward streaming protons and the persistent difference between their speeds as evidence that the source of the Earthward flow is moving tailwards onto magnetic field lines mapping to progressively higher polar latitudes as substorm recovery proceeds and we suggest that the source is the region of magnetic reconnection associated with a tailward retreating neutral line.

Field‐aligned currents in the Earth's magnetotail

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.

Rocket measurements of electric fields, electron density and temperature during different phases of auroral substorms

On 27 January 1979, three rocket payloads were launched from Kiruna, Sweden into different phases of two successive auroral substorrns. Among other experiments, the payloads carried the RIT double probe electric field experiments providing electric field, electron density and temperature data which are presented here. These data supported by rocket particle observations are discussed mainly in association with ground-based observations (magnetometer, TV) and very briefly with GEOS electric field data. The motions of the auroral forms as obtained from auroral pictures are compared with E × B /B 2 drifts and the currents calculated from the rocket electric field and density measurements with the equivalent current system deduced from ground-based magnetometer data (Scandinavian Magnetometer Array).

A nonthermal ion layer with high anisotropies at the plasmasheet boundary during substorm recovery

The energy spectra and anisotropies of protons and alpha particles have been investigated at the plasmasheet boundary during the recovery phase of geomagnetic substorms using the Max-Planck-Institut/University of Maryland sensor system on the ISEE-1 satellite [1]. The observations reveal the presence of a thin nonthermal layer of ∼60 keV/charge protons and alpha particles at the plasmasheet boundary, the particles streaming highly collimated in earthward direction. The alpha particle layer is confined within the proton layer. The main features of the layers can be explained in terms of an acceleration model proposed by Speiser [2] for the environment of a magnetic neutral line.

Magnetic storms, substorms and microsubstorms

The present conception of substorms is now under development within the scope of two basic models. In one of them (model 1) an active substorm phase is due to instability (plasma sheet reconnection) resulting from the accumulation of solar wind energy, transported to the magnetosphere, in the form of magnetotail energy. In model 2 the active phase requires no energy accumulation. In this paper a semi-empirical model is proposed where the active substorm phase comprises two periods. The first period (convection-type disturbances) corresponds to model 2, the second one (anticonvection-type disturbances) does not contradict model 1.

Time sequence analysis of flickering auroras, 1. Application of Fourier analysis

Using a technique that enables one to digitize the brightness of auroral displays from individual fields of a video signal, we have analyzed the frequency content of flickering aurora. Through the application of Fourier analysis to our data, we have found that flickering aurora contains a wide range of enhanced frequencies, although the dominant frequency enhancement generally occurs in the range 6–12 Hz. Each incidence of flickering that we observed was associated with increased radio wave absorption. Furthermore, we have found that flickering occurs in bright auroral surges, the occurrence of which is not limited to the ‘breakup’ phase of auroral substorms. Our results are interpreted in terms of a recently proposed theory of fluctuating double layers that accounts for a number of the observational features.

Observations of a nonthermal ion layer at the plasma sheet boundary during substorm recovery

We present measurements of the energy and angular distributions of energetic protons and alpha particles (≳30 keV/charge) in the geomagnetic tail. The measurements were made during the recovery phase of a geomagnetic substorm on April 19, 1978, with the Max‐Planck‐Institut/University of Maryland sensor system on the Isee 1 satellite. The measurements were also correlated with plasma observations made by the LASL/MPE instrument on Isee 1. The data reveal the presence of a thin nonthermal layer of protons and alpha particles at the plasma sheet boundary. The particles have their maximum flux at ∼60 keV/charge and are streaming highly collimated in the earthward direction. The alpha particle layer is confined within the proton layer. Many aspects of the observations are in agreement with an acceleration model near the neutral line proposed by Jaeger and Speiser (1974).

Simultaneous rocket probe, scintillation, and incoherent scatter radar observations of irregularities in the auroral zone ionosphere

Two rocket payloads carrying plasma density probes with high spatial resolution have been flown in the auroral zone during active conditions. Simultaneous Wideband satellite scintillation and Chatanika incoherent scatter radar observations were made in order to study the properties of high‐latitude irregularities and their effects on radio wave transmission. Unlike barium cloud striations and bottomside equatorial spread F, the observed power law dependence of the irregularities does not seem to be due to steepening of kilometer‐scale structures, rather, a turbulent process seems to occur. In addition the power law indexes determined both from the probe and from the scintillation measurements indicates an in situ one‐dimensional spectrum less steep than the k−2 value often reported. Both the probe and the scintillation data indicate absolute electron density fluctuations (Δn2e)½ of several times 109 m−3 during the expansion phase of an auroral substorm, with a layer thickness of several hundred kilometers. The observed S4 levels at VHF were in the range of 0.1–0.4. This level of scintillation, as well as the absolute density fluctuation levels and the power spectral density at the kilometer scale, are shown to be comparable with bottomside equatorial spread F. It is suggested that differences between the power spectral index in the present data set and the other ionospheric experiments mentioned above may be due to a highly conductive E layer and its effects upon the nonlinear evolution of irregularities. During another flight with lower magnetic activity but several bright auroral areas in the trajectory, much lower levels of absolute and relative density fluctuations were observed with a corresponding lower value for S4. Two very sharp changes in electron density were observed (e‐folding scales of 1.45 and 0.7 km) near the field line projected position of the auroral arcs. The associated density spectra were peaked at short wavelengths. The detection of very structured plasma within minutes of the poleward expansion phase of a substorm suggests that the F layer irregularities were formed in the precipitation event. On the other hand, evidence is also presented for production or enhancement of irregularities in the presence of horizontal density gradients which suggests that plasma instabilities also play a role in the production of auroral zone irregularities.