Quiet Auroral Arcs Research Articles

Problems with mapping the auroral oval and magnetospheric substorms.

Accurate mapping of the auroral oval into the equatorial plane is critical for the analysis of aurora and substorm dynamics. Comparison of ion pressure values measured at low altitudes by Defense Meteorological Satellite Program (DMSP) satellites during their crossings of the auroral oval, with plasma pressure values obtained at the equatorial plane from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite measurements, indicates that the main part of the auroral oval maps into the equatorial plane at distances between 6 and 12 Earth radii. On the nightside, this region is generally considered to be a part of the plasma sheet. However, our studies suggest that this region could form part of the plasma ring surrounding the Earth. We discuss the possibility of using the results found here to explain the ring-like shape of the auroral oval, the location of the injection boundary inside the magnetosphere near the geostationary orbit, presence of quiet auroral arcs in the auroral oval despite the constantly high level of turbulence observed in the plasma sheet, and some features of the onset of substorm expansion.

A survey of quiet auroral arc orientation and the effects of the interplanetary magnetic field

Using data from the THEMIS All‐Sky Imager array, we have carried out an extensive study of the orientation of quiet auroral arcs relative to the magnetic east‐west direction. We used over 7500 images of quiet auroral arcs that were collected during extended solar minimum and at various geomagnetic latitudes and longitudes. For each arc, we determined its “tilt” (the angle the arc makes with the local magnetic east‐west direction) and its “multiplicity” (whether or not the arc was part of a multiple‐arc system). We have found that at more equatorward latitudes, arc tilts are within σSD = ±7.7∘. We determined that both single‐ and multiple‐arc systems tend to tilt a few degrees to the south‐east prior to 23 magnetic local time (MLT) and to the north‐east afterward. This tilt appears to be more prominent at higher latitudes. We compared the auroral arc orientations to the mapping of equatorial contours of constant magnetic field strength into the ionosphere, where we used the T87 and T89 magnetic field models for quiet (Kp = 1,3) conditions for the mappings and to determine the constant equatorial magnetic field strength contours. We found that the MLT trends of the tilts are such that arc alignment appears to follow the constant magnetic field strength contours as projected into the ionosphere. We assert that the systematic dependencies of the orientation of auroral arcs indicate that arc morphology is governed by the large‐scale structure of the magnetosphere as opposed to localized processes within the ionosphere. In addition, we studied the effects of the interplanetary magnetic field (IMF) on the location in MLT of the reversal of the arc tilts. We found that negative IMF Bxand Byconditions cause the reversal location to shift duskward of 23 MLT. Alternately, a positive IMF Bx, coupled with a negative By, results in a shift in reversal location toward magnetic midnight. This behavior is consistent with that found in studies of the MLT distribution of substorm onsets.

Response of the Magnetosphere to Microscopic Effects in Auroral Arc Formation

A process of quiet auroral arc formation, in particular, the response of the magnetosphere to the effect of auroral energetic electrons, is studied using a holistic auroral simulation code. It consists of a three-dimensional magnetohydrodynamic code for a dipole magnetosphere-ionosphere coupling system and a one-dimensional electrostatic plasma particle code for auroral energetic electron production. Results of the holistic auroral simulation indicate that drastic variation in the ionospheric electric field, which is induced by auroral energetic electrons and propagates into the magnetosphere, influences the magnetospheric field-aligned current distribution. Furthermore, the characteristic time scale of this process is investigated.

Simulation of feedback instability in the coupled magnetosphere‐ionosphere system

Quiet auroral arcs formation has been investigated theoretically and numerically in a self‐consistent dynamic way. By using a three‐dimensional magneto‐hydro‐dynamics simulation of a dipole magnetosphere‐ionosphere coupling system, it is shown that multiple longitudinally striated structures of the ionospheric plasma density and the field‐aligned current are formed, resulting from nonlinear feedback instability. The areas where these structures appear are consistent with the prediction by the integrated feedback theory that includes the effects of the spatially non‐uniform electric field and non‐uniform plasma density. Effects of the difference of the field line lengths between the ionosphere and the magnetospheric equator over the auroral latitudes are also discussed on the feedback instability.

MHD–PIC connection model in a magnetosphere–ionosphere coupling system

A new simulation model is developed, in which the interaction between the macroscopic and microscopic plasma processes is able to be taken into account self-consistently. Magnetohydrodynamic (MHD) simulation and particle-in-cell (PIC) simulation are directly connected and performed simultaneously. This MHD–PIC connection model is first applied to the study of the quiet auroral arc formation process. It is an example of our challenge to create the holistic simulation framework, that is, the heterogeneous schemes that can be unified by the state-of-the-art numerical technique. In this paper, we will explain the algorithm of our new model and show the results which have been calculated on the Earth Simulator.

Response of the quiet auroral arc motion to ionospheric convection variations

Equatorward motion of quiet auroral arcs was studied together with the F region ionospheric plasma drift observed by the European Incoherent Scatter radar in Tromsø. Variations in the arc velocity lagged behind corresponding variations in the ionospheric convection velocity by 5–9 min. The same result was obtained from comparison of the arc velocity with variations in the eastward component of ground magnetic field. The variations in the arc and plasma drift velocities followed interplanetary magnetic field Bz variations. The observations indicate that the dawn‐dusk electric potential difference arises at the open polar cap magnetic field lines due to the solar wind ‐ magnetosphere interaction and then propagates to the closed magnetotail field lines through the ionosphere.

Solar cycle correlations of substorm and auroral occurrence frequency

Recent results of Newell et al. [1998a] suggest that the polar cap integrated auroral occurrence frequency is independent of the solar cycle phase. This paper utilizes auroral all‐sky camera observations from Northern Finland during 1974–1996 to show that although the total number of auroral observations shows only a weak solar cycle dependence, the number of quiet auroral arcs is anticorrelated with the sunspot number, indicating that active auroral forms are less common during sunspot minima. A local index for geomagnetic substorm occurrence frequency and intensity is developed. It is shown that in long‐term averages this index is highly correlated with the planetary aa index, and that both show strong correlation with the sunspot number. We further demonstrate a clear correlation between the active auroral forms and the local geomagnetic disturbances.

Instability of the magnetosphere-ionosphere convection and formation of auroral arcs

Abstract. In this paper we study an instability of the plasma moving towards the Earth near the inner plasma sheet boundary. We include both the interchange instability of the plasma sheet and the magnetosphere-ionosphere interaction instability caused by an effect of field-aligned currents (connected with electron precipitation) on ionospheric conductivity. The instability leads to the separation of steady-state magnetospheric convection into parallel layers. This instability may be responsible for the appearance of quiet auroral arcs inside region 2 of field-aligned currents flowing out of the ionosphere. This instability allows us to explain also the existence of crossing auroral arcs.

Self‐excitation of auroral arcs in a three‐dimensionally coupled magnetosphere‐ionosphere system

This paper presents the first full three‐dimensional dynamic simulation of auroral arc formation. The magnetospheric and ionospheric dynamics are represented by one‐fluid magnetohydrodynamic equations and two‐fluid weakly ionized plasma equations, respectively. The feedback coupling between magnetospheric Alfven waves and ionospheric density waves results in a spontaneous generation of longitudinally elongated striations of field‐aligned currents and ionospheric electron densities, which compare very well with many features of quiet auroral arcs.

The propagation of auroral hiss observed on the ground as deduced from direction-finding measurements

Based on the direction-finding results of auroral VLF hiss at Syowa (geomag. lat. −70°), an extensive ray tracing analysis, and an estimation of transmission loss in the simulated auroral ionosphere, the propagation characteristics of ground-based auroral hiss in the magnetosphere and ionosphere are deduced ; incoherent transmission of impulsive hiss with a wide frequency range (⩾ 100 kHz) from localized exit regions at the ionospheric level almost coincident with some localized regions of bright electron auroras; non-ducted propagation of continuous hiss with a narrow frequency range (⩽20 kHz) after emerging from a duct exit at higher altitudes (3000–5000 km); and coherent transmission from an exit region at the ionospheric level between Syowa and the location of a quiet auroral arc appearing poleward far from Syowa. Coupled with previous spacebased observations and model calculations of the auroral hiss power flux spectrum, we see that impulsive hiss emissions generated in a wide range of higher to low altitudes by the beam amplification are trapped in irregularities of decreased electron density outside the auroral arc at lower altitudes, propagate down to the ionosphere and are transmitted to the ground, and that after emerging from a duct exit at altitudes of 3000–5000 km, continuous hiss emissions are propagated in a non-ducted mode down to the ionosphere at different latitudes, corresponding to the initial wave normals at the duct exit, and are transmitted to the ground.

Full wave analysis of altitude profiles of auroral hiss observed by antarctic rocket.

Electromagnetic waves in the VLF range associated with auroral hiss were observed by the S-310JA-11 and -12 rockets launched from Syowa Station into quiet auroral arcs and breakup auroras, respectively. The altitude profile of electric field intensity observed by the S-310JA-12 rocket in the frequency range less than 5kHz showed a distinct peak at an altitude of 90km. The source location of the auroral hiss has been estimated so that the observed altitude profile of the hiss intensity, particularly around the altitude of 90km, coincides with the altitude profile calculated using the full wave analysis. It was found that the auroral hiss is most likely to be generated at an altitude about 700km. The intensity of auroral hiss around the altitude of 200km at a frequency of 4kHz observed by the rocket was 4.3×10-5V/m/√Hz, which is much stronger, by about 50dB, than that of auroral hiss observed on the ground at Syowa Station. This means that the k-vector direction of auroral hiss observed on the rocket is outside the transmission cone.

Structure of the auroral ionosphere observed by the antarctic rockets S-310JA-11 and -12.

Electron density profiles in the auroral ionosphere were measured by the impedance probe on board the S-310JA-11 and -12 rockets. The S-310JA-11 rocket was launched from Syowa Station into quiet auroral arcs (-2kR) on May 29, 1985, while the S-310JA-12 rocket was launched into active arcs (-14kR) on July 12, 1985. The S-310JA-11 data showed a typical altitude profile of the electron number density in an auroral arc under geomagnetically quiet condition, with a peak number density of 2×105cm-3 at 115km altitude. The S-310JA-12 data, which was obtained in bright auroras in the expansion phase of a substorm, showed a peak density of 1.2×106cm-3 at 101km altitude. On the downleg of the S-310JA-12 rocket, strong irregularities in the electron density distribution were observed at F region height in the active auroral arc region. It is pointed out that the ionospheric structure in the E region is produced mainly by precipitating particles while that in the F region seems to be largely affected by dynamical processes.

High‐frequency waves generated by auroral electrons

Measurements of marginally unstable electron distribution functions and high‐frequency plasma waves were made on a sounding rocket flight through a quiet auroral arc. The waves appeared near the electron plasma frequency and had a large parallel electric field component such that k∥ > k⊥. The appearance of these waves was correlated with the presence of marginally unstable parallel electron distributions. Analysis has shown that the waves were produced by ∂fe/∂ν∥ > 0 rather than the small ∂fe/∂ν⊥ > 0 features. Wave levels and growth rates inside the arc were small, and nonlinear wave‐wave and wave‐particle interactions appear to have been minimal.

Two‐dimensional quasi‐neutral description of particles and fields above discrete auroral arcs

This paper reports a full two‐dimensional numerical computation of the particle distributions, electric fields, and currents associated with a standard adiabatic model of a quiet auroral arc and inverted‐V potential. Both the region of auroral electron precipitation and the return current region are modeled; for the latter we assume a normal return current dominated by ambipolar E∥, with no substantial downward E∥. We go beyond earlier treatments of the adiabatic model (which either had no coupling between different field lines or treated all field lines as having essentially identical conditions along B) by specifying latitudinal variations in particle sources and parallel potential drops, which require every field line to be treated separately, coupled to other lines by current conservation. In particular, differential pitch angle anisotropies which drive upward E∥ are varied with latitude in order to accommodate a smooth transition to the return current region. In this transition regime the model predicts density enhancements on field lines at the edge of the inverted V that have only a small potential drop. We believe this is the first work to address the return current region with a realistic adiabatic model of the magnetosphere, including particle populations of both magnetospheric and ionospheric origin.

Quiet auroral arcs: Ionosphere effect of magnetospheric convection stratification

A detailed study of the mechanism of electromagnetic stratification of the large-scale stationary magnetospheric convection due to a friction of the convective flow in the ionosphere layer was performed. Magnetosphere-ionosphere interaction was taken into account by means of the effective boundary conditions on the ionosphere top and bottom boundaries including the actual height profile of charge particles velocity in the ionosphere. It has been shown that the magnetospheric convection is stratified into small-scale current sheets which are respective in the linear approximation to an oblique Alfvén wave. The dispersion equation was deduced for the Alfvén mode and its solution obtained determining the space-time scales and the increment of instability. The maximum increment is realized for the disturbances stretched along the convection velocity that is correspondent to the actual orientation of the auroral arcs. In the conditions of rapid growth of Alfvén velocity above the maximum of the ionosphere F layer, it was shown that small-scale disturbances with the transverse scales l ⊥ ≲ 1 km are localized at the altitudes up to several thousand kilometers whereas the large-scale stratification penetrate into the equatorial plane of the magnetosphere. A mechanism is proposed to intensify the parallel electric field acting at that stratification stage when the field-aligned currents in the Alfvén wave are sufficient to form abnormal resistance along geomagnetic lines of force.

Production of auroral zone E region irregularities by powerful HF heating

Production of ionospheric 1‐m irregularities in the auroral zone E region by high power ionospheric heating has been observed by means of the STARE auroral radar. Irregularities were produced at night, on eight occasions by using an O mode heating wave and on two occasions by using an X mode heating wave. The growth time of the irregularities is found to be approximately 30 s. Several physical possibilities to explain these findings are discussed, and a mechanism is suggested that is closely related to Sato's mechanism of quiet auroral arc formation.

A sounding rocket observation of an apparent wake generated parallel electric field

Sounding rocket Terrier‐Malemute 29.007 was launched over a 40 kR quiet auroral arc at 0813 UT on March 9, 1978. Payload instrumentation included a single‐axis double probe electric field detector. The reduced data from this detector are modulated by a square wave which is coherent with the diagnostic float‐bias cycle of the detector for the last ∼80 s of the flight when the analysis is carried out under the assumption that E · B = 0. This modulation may be removed by assuming that a time‐varying, vehicle‐generated apparent parallel field was present during the periods when the probes were floating. Three possible models for the effect were considered: a gradient in the nonthermal probe current, a gradient in plasma temperature near the rocket, and a variable dipole moment of the wake. There are significant problems with all these models. On balance, the third one appears to be the most likely. The observed effect may account for some previously reported measurements of low altitude parallel fields. It must be stressed that it cannot account for all such reports.

Electrostatic model of a quiet auroral arc

Given auroral electron and ion distribution functions as observed with satellites in inverted V events, we construct a self‐consistent electrostatic field distribution (both parallel and perpendicular to the earth's magnetic field). This field distribution is determined by (1) magnetic mirror forces which cause charge separation for species with different pitch angle distributions; (2) Poisson's equation, which gives the electric field in terms of the charge separation; (3) the ionospheric physics of charge and current conservation, coupled with precipitation sources and recombination losses; and (4) particle distribution functions specified at the equator (and for thermal and backscattered plasma, at the ionosphere). We assume that equatorial particle distribution functions depend on L only through the dependence of the electrostatic potential on L; an assumed factorized form for the potential allows this L dependence to be simply parameterized. These ingredients combine to yield a self‐consistent latitudinal scale length of some tens of kilometers, typical for quiet arcs. A variety of particle and field data at and below S3‐3 altitude are fit semiquantitatively with the model. We do not consider in this paper physical processes which act in the equatorial magnetosphere, although they could well be connected to the processes we do consider through some sort of feedback.

Ionospheric electrical currents in the late evening plasma flow reversal

An instrumented sounding rocket was launched from Andøya, Norway, in January 1977, in the late evening auroral oval. Its trajectory took it northward over a quiet auroral arc and over a region of plasma flow reversal. Horizontal currents were inferred from magnetic field measurements; they were also calculated from the measured electric field and conductivities derived from energetic particle influxes. South of the reversal region the sheet current density of the eastward electrojet was found to be ∼0.1 A/m, while north of the reversal the current density was less than 0.05 A/m. The currents were enhanced in the arc. Current continuity considerations in the meridian plane implied downward field‐aligned currents at the southern edge of the arc and upward field‐aligned currents at the northern edge.

A quasi-self-consistent axially symmetric model for the growth of a ring current through earthward motion from a pre-storm configuration

A quasi-self-consistent axially symmetric model of a storm-time ring current that has evolved quasi-adiabatically through inward motion from a prestorm state is presented in which the disturbance field as a function of geocentric distance, r, in the equatorial plane (including the value, H, at r = 0 from which Dst can be found), the beta of the plasma as a function of r, and the ring current magnetic moment are all given in terms of analytic expressions, having only two independent ring current parameters: the geocentric distance to the inner edge of the ring current, R, and the distance at which the beta of the plasma is unity, k—a constant of the model. The model is used to find H as a function of R at constant k, which corresponds to the growth of Dst as the ring current moves earthward, and to illustrate how the radial profile of the disturbance field varies also as the ring current moves earthward. Comparison with published S 3-A storm-time, particle and field data shows reasonable agreement. The model is also able to fit the empirically determined relationship between Dst and the equatorward extension of quiet auroral arcs. This result applied to the great magnetic storm of September 1859 predicts a Dst of the order of 2000 nT, a value for which the sparse magnetometer data for that storm offer some support. Because the model (quasi) self-consistently determines the particle distribution function by allowing it to (quasi) adiabatically evolve in a field of its own making, the model avoids difficulties encountered by previous ring current models in which the particle distribution function had to be assumed. For example, the ring current magnetic moment is not limited to values less than the geomagnetic dipole moment, and null points in the field do not develop even in very high beta situations.