Diffuse Auroral Zone Research Articles

Optical observations of large-scale undulations in the 23rd cycle of solar activity

A statistical analysis of observations of large-scale undulations during the 23rd cycle of solar activity was performed using optical data from two stations: Tixie (71.6°N, 128.9°E) and Zhigansk (66.8°N, 123.4°E). The total number of events recorded was 54 (43 events at Tixie and 11 at Zhigansk). The complete list of observed events is presented. The occurrence frequency of eveningside (17–23 LT) undulations during the solar activity growth (1999) and decline (2003–2005) phases tends to increase. Large-scale undulations were shown to be generated both on the equatorward boundary of the diffuse auroral zone and inside the diffuse zone, which does not necessarily occur during magnetic storms.

Spatial distribution of auroral precipitation during storms caused by magnetic clouds

The global pattern of auroral precipitation and dynamics of precipitation boundaries during three different intensity magnetic storms driven by magnetic clouds were investigated. For the aim of the research, the empirical model (http://pgia.ru/lang/en/webapps/) in which the boundary locations of the auroral precipitation depend on the geomagnetic activity expressed by the AL- and Dst indices was used. The locations of the boundaries derived from DMSP F10–F15 spacecraft observations were compared to those obtained in the model and displayed reasonable agreement. We find a significant displacement to the lower latitudes of the diffuse auroral zone (DAZ) and auroral oval precipitation (AOP) region with the increase of magnetic activity. The planetary pattern of auroral precipitation indicated different dawn–dusk widening of the DAZ and AOP region (asymmetry) during both main and recovery phases of magnetic storms. Differences in the dawn–dusk widening (i.e., asymmetry) of the DAZ and AOP zone during magnetic storms appear to be sensitive to Dst, where the DAZ widens in the morning only, while the AOP widens in the evening under all Dst intensities, and widens significantly in the morning also for Dst<−100nT. The average energy of precipitating electrons in both MLT sectors and both zones was estimated and compared with DMSP spacecraft data.

Resonant scattering of plasma sheet electrons leading to diffuse auroral precipitation: 1. Evaluation for electrostatic electron cyclotron harmonic waves

[1] Using statistical wave power spectral profiles obtained from CRRES and the latitudinal distributions of wave propagation modeled by the HOTRAY code, a quantitative analysis has been performed on the scattering of plasma sheet electrons into the diffuse auroral zone by multiband electrostatic electron cyclotron harmonic (ECH) emissions near L = 6 within the 0000–0600 MLT sector. The results show that ECH wave scattering of plasma sheet electrons varies from near the strong diffusion rate (timescale of an hour or less) during active times with peak wave amplitudes of an order of 1 mV/m to very weak scattering (on the timescale of >1 day) during quiet conditions with typical wave amplitudes of tenths of mV/m. However, for the low-energy (∼100 eV to below 2 keV) electron population mainly associated with the diffuse auroral emission, ECH waves are only responsible for rapid pitch angle diffusion (occasionally near the limit of strong diffusion) for a small portion of the electron population with pitch angles αeq 70°. Computations of the bounce-averaged coefficients of momentum diffusion and (pitch angle, momentum) mixed diffusion indicate that both mixed diffusion and energy diffusion of plasma sheet electrons due to ECH waves are very small compared to pitch angle diffusion and that ECH waves have little effect on local electron acceleration. Consequently, the multiple harmonic ECH emissions cannot play a dominant role in the occurrence of diffuse auroral precipitation near L = 6, and other wave-particle interaction mechanisms, such as whistler mode chorus-driven resonant scattering, are required to explain the global distribution of diffuse auroral precipitation and the formation of the pancake distribution in the inner magnetosphere.

Seasonal and UT variations of the position of the auroral precipitation and polar cap boundaries

The data of the DMSP F7 spacecraft are used for studying the influence of the geomagnetic dipole tilt angle on the latitudinal position of auroral precipitation boundaries in the nighttime (2100–2400 MLT) and daytime (0900–1200 MLT) sectors. It is shown that, in the nighttime sector, the high-latitude zone of soft diffuse precipitation (SDP) and the boundary of the polar cap (PC) at all levels of geomagnetic activity are located at higher and lower latitudes relative to the equinox period in winter and summer, respectively. The position of boundaries of the diffuse auroral precipitation zone (DAZ) located equatorward from the auroral oval does not depend on the season. In the daytime sector, the inverse picture is observed: the SDP precipitation zone takes the most low-latitude and high-latitude positions in the winter and summer periods, respectively. The total value of the displacements from winter to summer of both the nighttime and daytime boundaries of the PC is ∼2.5°. A diurnal wave in the latitudinal position of the nighttime precipitation boundaries is detected. The wave is most pronounced in the periods of the winter and fall seasons, is much weaker in the spring period, and is almost absent in summer. The diurnal variations of the position of the boundaries are quasi-sinusoidal oscillations with the latitude maximum and minimum at 0300–0500 and 1700–2100 UT, respectively. The total value of the diurnal displacement of the boundaries is ∼2.5° of latitude. The results obtained show that, undergoing seasonal and diurnal variations, the polar cap is shifted as a whole in the direction opposite to the changes in the tilt angle of the geomagnetic dipole. The seasonal displacements of the polar cap and its diurnal variations in the winter period occur without any substantial changes in its area.

Electric fields and large-scale undulations in the evening sector of the diffuse auroral zone

The synchronous observations of strong electric fields and large-scale undulations observed on December 12, 2004, in the evening sector of the diffuse auroral zone 0900-1000 UT (~1700-1800 MLT) have been analyzed. The appearance of strong northward electric field at ~0900 UT was almost simultaneously registered at Tixie Bay ionospheric station (71.6° N, 128.9° E, L =, 5.6) and on the DMSP F15 satellite. At 0910-1000 UT, the all-sky TV camera at Tixie Bay and the DMSP satellites (F13, F14, and F15) registered eight undulations propagating westward at a velocity of 0.7—0.8 km/s. The undulation parameters registered during the TV observations agree with the satellite measurements. The distinctive feature of the analyzed event consists in that an intense electric field and undulations were localized within the diffuse zone in the region of increased precipitation of keV electrons. A comparison of the ground-based and satellite measurements made it possible to draw the conclusion on the necessary conditions for formation of diffuse undulations.

10.1007/s11478-008-2004-5

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.

A study of fine structure of diffuse aurora with ALIS-FAST measurements

Abstract. We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone.

Features of morning-time auroras during SC

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.

Features of the planetary distribution of auroral precipitation characteristics during substorms

A planetary pattern of substorm development in auroral precipitation has been constructed on the basis of the F6 and F7 satellite observations. The behavior of the auroral injection boundaries and characteristics of precipitating electrons in various precipitation regions during all phases of a statistically mean magnetospheric substorm with an intensity of AL ∼ −400 nT at a maximum is considered in detail. It is shown that during a substorm, the zone of structured auroral oval precipitation AOP and the diffuse auroral zone DAZ are the widest in the nighttime and daytime sectors, respectively. In the daytime sector, all precipitation regions synchronously shift equatorward not only at the origination phase but during the substorm development phase. The strongest shift to low latitudes of the daytime AOP region is observed at a maximum of the development phase. As a result of this shift, the area of the polar cap increases during the phases of substorm origination and development. It is shown that the average position of the precipitation boundaries and the energy fluxes of precipitating electrons at each phase are linearly related to the intensity of a magnetic disturbance. This makes it possible to develop a model of auroral precipitation development during each phase of substorms of any intensity.

First current density measurements in the ring current region using simultaneous multi-spacecraft CLUSTER-FGM data

Abstract. The inner magnetosphere's current mapping is one of the key elements for current loop closure inside the entire magnetosphere. A method for directly computing the current is the multi-spacecraft curlometer technique, which is based on the application of Maxwell-Ampère's law. This requires the use of four-point magnetic field high resolution measurements. The FGM experiment on board the four Cluster spacecraft allows, for the first time, an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density. This technique requires, however, a careful study concerning all the factors that can affect the accuracy of the J estimate, such as the tetrahedral geometry of the four spacecraft, or the size and orientation of the current structure sampled. The first part of this paper is thus providing a detailed analysis of the method accuracy, and points out the limitations of this technique in the region of interest. The second part is an analysis of the ring current region, which reveals, for the first time, the large latitudinal extent of the ring current, for all magnetic activity levels, as well as the latitudinal evolution of the perpendicular (and parallel) components of the current along the diffuse auroral zone. Our analysis also points out the sharp transition between two distinct plasma regions, with the existence of high diamagnetic currents at the interface, as well as the filamentation of the current inside the inner plasma sheet. A statistical study over multiple perigee passes of Cluster (at about 4 RE from the Earth) reveals the azimuthal extent of the partial ring current. It also reveals that, at these distances and all along the evening sector, there isn't necessarily a strong dependence of the local current density value on the magnetic activity level. This is a direct consequence of the ring current morphology evolution, as well as the relative positioning of the spacecraft with respect to the bulk of the ring current. It also proves the existence of a substantial ring current at these distances, all over the evening and the post-midnight sector. Keywords. Magnetospheric physics (Current systems; Energetic particles, trapped; Magnetospheric configuration and dynamics)

Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents

Abstract. Abstract. Simultaneous DMSP F7 and Viking satellite measurements of the dawnside high-latitude auroral energy electron and ion precipitation show that the region of the low and middle altitude auroral precipitation consists of three characteristic plasma regimes. The recommendation of the IAGA Working Group IIF/III4 at the IAGA Assembly in Boulder, July 1995 to decouple the nomenclature of ionospheric populations from magnetospheric population is used for their notation. The most equatorial regime is the Diffuse Auroral Zone (DAZ) of diffuse spatially unstructured precipitating electrons. It is generated by the plasma injection to the inner magnetosphere in the nightside and the subsequent drift plasma to the dawnside around the Earth. Precipitating particles have a hard spectrum with typical energies of electrons and ions of more than 3 keV. In the DAZ, the ion pitch-angle distribution is anisotropic, with the peak near 90°. The next part is the Auroral Oval (AO), a structured electron regime which closely resembles the poleward portion of the night-side auroral oval. The typical electron energy is several keV, and the ion energy is up to 10 keV. Ion distributions are pre-dominantly isotropic. In some cases, this plasma regime may be absent in the pre-noon sector. Poleward of the Auroral Oval, there is the Soft Small Scale Luminosity (SSSL) regime. It is caused by structured electron and ion precipitation with typical electron energy of about 0.3 keV and ion energy of about 1 keV. The connection of these low-altitude regimes with plasma domains of the distant magnetosphere is discussed. For mapping of the plasma regimes to the equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1995) and the conceptual model by Alexeev et al. (1996) are used. The DAZ is mapped along the magnetic field lines to the Remnant Layer (RL), which is located in the outer radiation belt region; the zone of structured electrons and isotropic ion precipitation (AO) is mapped to the dawn periphery of the Central Plasma Sheet (CPS); the soft small scale structured precipitation (SSSL) is mapped to the outer magnetosphere close to the magnetopause, i.e. the Low Latitude Boundary Layer (LLBL). In the near-noon sector, earthward fluxes of soft electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion energies decrease with increasing latitude. The plasma spectra of the DRA regime are analogous to the spectra of the Plasma Mantle (PM). In the dawn sector, the large-scale field-aligned currents flow into the ionosphere at the SSSL latitudes (Region 1) and flow out at the AO or DAZ latitudes (Region 2). In the dawn and dusk sectors, the large-scale Region 1 and Region 2 FAC generation occurs in different plasma domains of the distant magnetosphere. The dawn and dusk FAC connection to the traditional Region 1 and Region 2 has only formal character, as FAC generating in various magnetospheric plasma domains integrate in the same region (Region 1 or Region 2). In the SSSL, there is anti-sunward convection; in the DAZ and the AO, there is the sunward convection. At PM latitudes, the convection is controlled by the azimuthal IMF component (By ). It is suggested to extend the notation of the plasma pattern boundaries, as proposed by Newell et al. (1996), for the nightside sector of the auroral oval to the dawn sector.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; plasma convection)

Stationary magnetospheric convection on November 24, 1981. 1. A case study of &amp;quot;pressure gradient/minimum-&amp;lt;i&amp;gt;B&amp;lt;/i&amp;gt;&amp;quot; auroral arc generation

Abstract. We present two case studies in the night and evening sides of the auroral oval, based on plasma and field measurements made at low altitudes by the AUREOL-3 satellite, during a long period of stationary magnetospheric convection (SMC) on November 24, 1981. The basic feature of both oval crossings was an evident double oval pattern, including (1) a weak arc-type structure at the equatorial edge of the oval/polar edge of the diffuse auroral band, collocated with an upward field-aligned current (FAC) sheet of \\sim1.0 µA m-2, (2) an intermediate region of weaker precipitation within the oval, (3) a more intense auroral band at the polar oval boundary, and (4) polar diffuse auroral zone near the polar cap boundary. These measurements are compared with the published magnetospheric data during this SMC period, accumulated by Yahnin et al. and Sergeev et al., including a semi-empirical radial magnetic field profile BZ in the near-Earth neutral sheet, with a minimum at about 10-14 RE. Such a radial BZ profile appears to be very similar to that assumed in the "minimum- B/cross-tail line current" model by Galperin et al. (GVZ92) as the "root of the arc", or the arc generic region. This model considers a FAC generator mechanism by Grad-Vasyliunas-Boström-Tverskoy operating in the region of a narrow magnetic field minimum in the near-Earth neutral sheet, together with the concept of ion non-adiabatic scattering in the "wall region". The generated upward FAC branch of the double sheet current structure feeds the steady auroral arc/inverted-V at the equatorial border of the oval. When the semi-empirical BZ profile is introduced in the GVZ92 model, a good agreement is found between the modelled current and the measured characteristics of the FACs associated with the equatorial arc. Thus the main predictions of the GVZ92 model concerning the "minimum-B" region are consistent with these data, while some small-scale features are not reproduced. Implications of the GVZ92 model are discussed, particularly concerning the necessary conditions for a substorm onset that were not fulfilled during the SMC period.Key words. Magnetospheric physics (auroral phenomena; magnetospheric configuration and dynamics; plasma sheet).

Relationship of the ionospheric convection reversal to the hard auroral precipitation boundary

Plasma drift and particle measurements from the DMSP spacecraft for the three Geospace Environmental Modeling (GEM) campaign periods (January 27–29, March 28–29, and July 20–21, 1992) have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0.45 keV. This boundary, named the hard auroral precipitation boundary (or the HAP boundary) in this paper, is often regarded as the ionospheric footprint of the boundary between the central plasma sheet (CPS) and the boundary plasma sheet (BPS). By examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large‐scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the HAP boundary. However, exceptions are found in the early morning sector between 0300 and 0600 MLT, where the convection reversal is sometimes (in 30% of the DMSP crossings) displaced poleward relative to the HAP boundary. It is shown that the shape of the region bordered by the HAP boundary can be roughly represented by a circle, whose size is influenced by the IMF Bz component. There is roughly a linear correlation between the diameter of this circle and the cross‐polar‐cap potential drop, with the best correlation coefficient of 0.65 for winter season. Our study suggests that the HAP boundary corresponds to the magnetospheric boundary between the quasi‐dipolar region and the region with more stretched field lines, and the source of the region 1 field‐aligned current is located near the HAP boundary. A By‐dependent shift of the HAP boundary with respect to the noon‐midnight meridian is also found. In the northern hemisphere, the shift is dawnward for positive By and duskward for negative By in the southern hemisphere, the shift is opposite to that in the northern hemisphere.

Field-aligned flows of ionospheric plasma in the magnetosphere

Data are presented from occasions when one of the detectors of the Suprathermal Plasma Analyser on GEOS-2 observed along the magnetic field direction. Electrons were seen with larger fluxes along the field-aligned direction than at 80° to it. One particular class of these events was identified on the night-side when electrons in the energy range 10's to 100's eV were seen. These electrons of ionospheric origin may be associated with suprathermal electron bursts or the permanent power law population of the diffuse auroral zone.

Detection of suprathermal ionospheric O + ions inside the plasmasphere

The plasma diagnostic experiments on the AUREOL-3 satellite have revealed flows of low energy 0 + ions deep inside the night plasmasphere during a large substorm. Flux gradients of the 0 + ions were accompanied by enhancements of ELF electric field noise. The appearance of suprathermal ions at L ⋍ 2.5 – 3 is interpreted within the framework of electrostatic ion-cyclotron acceleration of ionospheric ions in the diffuse auroral zone /12/ followed by a radial displacement of these ions inside the plasmasphere driven by azimuthal electric fields during substorm activity. Electrostatic oscillations observed inside the plasmasphere are apparently associated with gradient instability at the sharp boundaries of suprathermal ion flows.

A theoretical approach to the morphology and the dynamics of diffuse auroral zones

Direct observations of the high‐latitude ionosphere have established the continuous presence of large‐scale emissions, referred to as diffuse auroras. Neglecting localized structures, such as discrete arcs, we focus upon a quantitative description of the effects of large‐scale convection on diffuse precipitation. The transport of 100 eV‐10 keV electrons from the geomagnetic tail earthward by the convection electric field, and their pitch angle diffusion into the loss cone by wave‐particle interactions are believed to be the main cause of diffuse auroral electron precipitation. A self‐consistent treatment balancing wave generation and particle diffusion appears to be still beyond the present state of the art. For the main purpose of diffuse precipitation modeling, we propose a simplified approach to this problem and test its validity against direct observations of the location and dynamics of diffuse auroral emissions. Using basically the strong pitch angle diffusion limit in the way proposed by Kennel [1969], we derive a set of fluid equations describing the three‐dimensional transport of plasma‐sheet electrons. Their integration provides the latitude and local time distribution of precipitation fluxes and characteristic energies at the top of the ionosphere as a function of the large‐scale dawn to dusk electrostatic potential drop. The calculated expansion of the auroral oval with magnetic activity, deduced from our model, approaches the experimental results. So this simplified theoretical study permits us to reproduce and explain the main characteristics of the diffuse auroral zone. However, for high values of magnetic activity, the theoretical results are found to be shifted poleward relative to the observations. This suggests that the assumption of strong pitch angle diffusion overestimates the efficiency of wave‐particle interactions.

Correlated observations of the equatorward diffuse auroral boundary

The equatorward boundary of the diffuse auroral zone, as viewed via the 6300‐Å (O(¹D)) emission line, has been examined by using ground‐based photometer data. These observations are supplemented with in situ measurements of ionospheric parameters by satellite‐borne instrumentation within the photometer field of view. We find high spatial correlation among 6300‐Å emission, low‐energy (E &lt;1 keV) electron precipitation, and the high‐latitude recovery of ambient electron density which constitutes the poleward cliff of the main ionospheric trough. Time resolution afforded by this ground‐based optical method is especially suited for monitoring the ionospheric signature of this low‐energy particle precipitation over broad longitudes. A model is developed of the statistical equatorward boundary of the diffuse aurora as a function of magnetic local time and magnetic activity (as monitored by Kp index). The local time dependence of this boundary exhibits a latitudinal morphology similar to that of the earthward equatorial plasma sheet—that is, asymmetric about local midnight and skewed toward lesser invariant latitude values within the morning sector.

Diffuse Jovian aurora influenced by plasma injection from Io

Cyclotron resonant instability of Jovian energetic electrons is dramatically enhanced as a consequence of the lower resonant electron energy (Eres ⧜ B²/8πN) within the equatorial high density plasma torus surrounding the orbit of Io. This mechanism accounts adequately for the continuous broad band whistler mode waves observed within the Io torus on Voyager I. The higher energy resonant electrons (∼ 100 keV to 1 MeV) can be scattered by the observed low frequency waves (≲ 1 kHz) at a rate as high as 10 percent of the limit attainable under strong pitch‐angle diffusion; the corresponding energetic electron lifetimes are comparable to a day. This is much shorter than any realistic estimates for radial diffusion transport near Io. Thus an efficient local acceleration process is required to continually replenish the precipitating relativistic electrons. Concomitant energy deposition into the Jovian atmosphere is estimated to be ∼5 to 15 ergs cm−2 sec−1 over a broad invariant latitude range mapping from the Io plasma torus (65° ≲ Δ ≲ 75°). This intense precipitation should both provide a dominant source of middle atmospheric ionization and excite a continuous band of diffuse auroral emission. In the absence of significant extinction, H Lyα intensity is estimated to be 30 to 100 kR in approximate agreement with the Voyager Extreme Ultraviolet observations. Total power dissipation over the diffuse auroral zone should be at least 1013 watts. Because the cyclotron resonant scattering process is strongly influenced by the ambient equatorial thermal plasma density we suggest that the diffuse Jovian aurora should be influenced by the variable volcanic activity on Io which is thought to be an important source of plasma.