Nighttime Auroral Zone Research Articles

The history of Andøya Rocket Range

Abstract. Just 5 years after Sputnik, on 18 August 1962, Norway launched the first sounding rocket from Andøya in northern Norway. The establishment of Andøya Rocket Range (ARR), in the Arctic and right in the middle of the night-time auroral zone, gave the scientists unique opportunities for studies of the complex processes in the auroral ionosphere and upper atmosphere. In close cooperation with the users, ARR gradually developed its technical and scientific infrastructure and is now one of the world's leading observatories in this field. ARR has also established a launch site at Svalbard, and sounding rockets from both ranges can reach far into the Arctic to study the cusp region and the daytime aurora. The ground-based instruments comprise sophisticated radars and lidars as well as passive instruments. ARR also plays an active role in space education. In 2014 Andøya Rocket Range changed its name to Andøya Space Center (ASC; https://www.andoyaspace.no, last access: 23 November 2018). This change reflects the fact that the activities now comprise much more than sounding rocket launches. ASC is an important company both nationally and in the local community of Andenes. ASC now has a staff of 95 and an annual turnover of NOK 150 million.

Winter anomaly of the E-layer critical frequency in the nighttime auroral zone

Analysis of the annual variation of the E-layer critical frequency median foE in the nighttime (22−02 LT) auroral zone by the data of several stations of the Northern Hemisphere has shown the median maximum in winter and minimum in summer, even though the summer contribution of solar radiation to foE is greater. Thus, a new phenomenon was discovered—an foE median winter anomaly in the nighttime auroral zone. Its amplitude (ratio of winter to summer foE figures) can reach 10–15%; however, this anomaly was weakly expressed and statistically insignificant at particular stations located in the auroral zone. The winter anomaly is more distinct for foE avr, the median of the E-layer critical frequency foE caused by the auroral source of atmospheric ionization, i.e., excluding the solar radiation contribution to foE. For foE avr, the amplitude of the winter anomaly can reach 15–20%. Based on the qualitative analysis, it has been found that foE winter anomaly is stipulated by the winter/summer asymmetry of energy flow of accelerated electrons, which induce discrete aurorae in the nighttime auroral zone.

Locations of auroral kilometric radiation bursts inferred from multispacecraft wideband Cluster VLBI observations. 1: Description of technique and initial results

The Cluster Wideband Data instrument has been used to determine the locations of auroral kilometric radiation (AKR) using very long baseline interferometry. The technique involves cross‐correlating individual AKR bursts from all six Cluster baselines using time and frequency filtered waveforms. The resulting differential delay peaks are used to determine source locations with an uncertainty as small as 500 km in a plane perpendicular to the source‐spacecraft line of sight or about 200 km when the burst position is projected onto the auroral zone along the magnetic field lines passing through the source. The uncertainty along the line of sight is much larger, but this is mitigated by assuming that the emission arises from a height corresponding to the electron gyrofrequency. We report the locations of over 1700 individual AKR bursts during six observing epochs between 10 July 2002 and 22 January 2003 when the Cluster constellation was high above the Southern or Northern Hemisphere. In general we find that the AKR burst locations lie along magnetic field lines which map onto the nighttime auroral zone as expected from previous AKR studies. For the three observing epochs viewing the Northern Hemisphere, there is a strong tendency for AKR burst locations to be centered within the auroral oval and in the evening sector. The Southern Hemisphere burst locations favor magnetic local midnight to early morning and have somewhat higher invariant magnetic latitudes. The distribution of AKR auroral footprint locations at each epoch had a overall spatial scale between 1000 and 2000 km, much larger than the positional uncertainty of an individual AKR burst location magnetic footprint, but a small fraction of the auroral oval. This indicates that on a timescale of 1–3 hours conditions for suitable generation of AKR emission are found on only a fraction of all magnetic field lines crossing through the auroral oval. For two of the six epochs, there was a significant drift in the mean location of AKR activity over a period of 1–2 hours. The drift was predominantly in latitude at one epoch and in longitude at the other, with average drift speed V ∼ 80–90 m s−1 at the AKR emission location.

Analysis of substorm disturbances in the night time auroral zone

Based on magnetic and ionospheric observations, and on such of optical emission in a meridional chain of stations near 90°E extending from L = 4 to L = 12, the high-latitude ionosphere was investigated during quiet and moderately disturbed day. In substorm the intensity and hardness of precipitating fluxes in the Harang discontinuity was found increased. In evening and near-midnight hours substorms are preceded by the start of an equatorward displacement of the E-region sporadic ionization zone. After that, the zone expands abruptly poleward, with a concurrent increase in its electron density. A discrete precipitation band lies inside the diffuse precipitation zone of low-energy electron fluxes, whose boundary can be determined both from the type of reflections on ionograms and from the background emission level. The maximum of the westward auroral electrojet lies not within the region of maximum conductivity but somewhat farther to the north.

Substorm activity precursors in the dayside magnetic perturbations

Magnetic data from a meridional chain of stations in Greenland and AL-indices of magnetic activity have been used to study the relationship between magnetic perturbations in the dayside cleft region and substorm activity in the night-time auroral zone. The analysis of 14 substorms, isolated and prolonged, has shown that intensification of westward currents in the postnoon sector of the cleft precedes or accompanies substorm development in the night-time auroral zone. Westward currents appear in the northern cleft as substorm precursors even under the adverse influence of the IMF positive B y component. These currents trend to extend in the prenoon sector. To explain the relationship between the cleft currents and auroral electrojet the connection between neutral layer currents and noon Birkeland currents is proposed. This connection can be realized by means of the source region acting just inside the daytime magnetopause owing to stationary reconnection of geomagnetic field and IMF, the source region flowing downstream to the tail magnetopause.

Structure of auroral zone phenomena from the data of meridional chains of stations: magnetic disturbances in the nighttime auroral zone and auroras

A new method of compulation of the equivalent currents using meridional chain stations' magnetic data is proposed to study the auroral electrojet structure. The correctness and resolving power of the method are estimated by test calculations. On the basis of computer simulation studies, it is shown that the method may be applied in practice when variations in the geomagnetic H-components are sufficiently larger than those in the D-component. Using the method, a fine structure of the westward electrojet was found. This structure is the same at two longitudinally separated meridians (in Scandinavia and the Kola peninsula) and seems to be associated with auroral features. The conclusion is made that the small-scale field-aligned and ionospheric currents with a width of less than 0.5 are not displayed in magnetic observations made on the ground against the effects of larger-scale structures.

Velocity‐dispersed ion beams in the nightside auroral zone: AUREOL 3 observations

Two years of data from the AUREOL 3 satellite have been used to study in detail the energetic ion beams which precipitate at low altitudes (400–2000 km) near the poleward boundary of the nighttime auroral zone. Eighty 2–20 keV events have been identified and examined; they typically display an energy‐latitude dispersion in the auroral projection of the boundary plasma sheet, with the more energetic ions precipitating at less than 0.5° invariant latitude from the polar cap boundary. Their occurrence maximum is found in the 20–04 MLT sector and is independent of the magnetic activity measured by the AE index. In all the events studied, the ion fluxes are less than 107 ions cm−2 s−1 sr−1 keV−1, and mass analysis demonstrates that they are mainly composed of energetic H+ ions. All the experimental facts are consistent with an ion acceleration by the dawn‐dusk electric field in the neutral sheet with a weak magnetic field normal component Bz in the distant magnetotail at an inferred distance of 50–100 RE. The observed velocity‐latitude ion segregation is the result of the E×B drift of the ion beams up to the ionosphere (geomagnetic filter effect) superimposed upon the intrinsic dispersive effect of the nonadiabatic energization mechanism, when a realistic dependence of Bz on downtail distance is adopted. Finally, from the striking similarity of the experimental ion distributions we believe that the ion beams observed at low altitudes close to the polar cap boundary are the signature in the auroral ion precipitation of the earthward flowing ion beams often detected along the plasma sheet boundary layer.

Classification of auroral precipitation fluxes by characteristic parameters and their effects on the coupling of the precipitation to the ambient ionosphere

The spectral shapes of the precipitating auroral electron flux spectra are quantified by an automated fitting procedure which represents each flux spectrum as a superposition of Maxwellian and Gaussian partial fluxes. This makes it possible to represent each spectrum by a set of characteristic parameters which describe the shape of that spectrum. A set of inverted-V events observed by the Low-Energy Plasma Instrument on Dynamics Explorer 2 near the fall 1981 equinox, has been analyzed. The distribution of the peak inverted-V energies in magnetic local time and invariant latitude has been obtained, and it is shown that by far the highest peak energies occur in the range 65 < IL < 70 and 18 < MLT < 24 hours. It is also shown how the difference in spectral characteristics of the precipitation fluxes between the cusp and the nighttime auroral zone determines the thermal coupling of the precipitation to the ambient ionosphere.

DE 1 observations of type 1 counterstreaming electrons and field‐aligned currents

In this paper we report that the Dynamics Explorer 1 satellite frequently detected type 1 counterstreaming electrons in the region of field‐aligned currents at high altitudes in the nighttime auroral zone. Type 1 counterstreaming electrons are generally detected at energies below a few hundred electron volts in association with energetic (&gt; keV) precipitating electron fluxes. The plasma and magnetic field measurements from DE 1 are used to investigate the relative contributions of energetic precipitating electron fluxes and counterstreaming electrons to the field‐aligned current density. The current density is computed by integrating the two‐dimensional electron distribution functions in two energy ranges: 18–235 eV and 18–10,000 eV. In the case of downward current, low‐energy (18 &lt; E &lt; 235 eV) electrons are found to carry more than 40% of the total plasma current density integrated above 18 eV, while higher‐energy electrons carry the rest. In the case of upward currents, less than 50% of the total plasma current density integrated above 18 eV is found to be contributed by low‐energy (18 &lt; E &lt; 235 eV) electrons. In general, the observed pitch angle distributions show that fluxes of counterstreaming electrons are enhanced at both small (&lt;40°) and large (&gt;140°) pitch angles. The energy spectra of counterstreaming electrons in the field‐aligned directions are frequently found to be characterized by Maxwellian functions and occasionally by spectral peaks. It is thus concluded that counterstreaming electrons contribute to field‐aligned current density mostly because of imbalances in the flux enhancement in the upward and downward directions. The Scandinavian Twin Auroral Radar Experiment radar simultaneous observations indicated that the field‐aligned current observed by DE 1 was closed through Pedersen currents in the ionosphere.

Alouette‐ISIS radio wave studies of the cleft, the auroral zone, and the main trough and of their associated irregularities

The principal high‐latitude ionospheric features such as the cleft, the nighttime auroral zone, the main trough and their associated irregularities, as observed with the Alouette/ISIS topside sounders, are reviewed. These features can be clearly distinguished in Alouette 1 electron density contour plots as a function of latitude and height below 1000 km. Alouette 2 data recorded near apogee (3000 km) show the location of the light‐ion (H+ and He+) trough as a function of local time and show it to be more closely related to the plasmapause than is the main trough. An attempt is made to clarify the location and terminology of troughlike structures. Regions of small‐scale irregularities (tens of meters) are associated with the high‐latitude ionospheric features. These irregularities are responsible for the presence of aspect‐sensitive spread F on topside ionograms. The most intense spread is found in the cleft region. Spread F regions usually have a sharp equatorward boundary. Some recent unpublished work by the author indicates that the spread F boundary for the nighttime auroral zone tends to be located several degrees equatorward of the keV electron precipitation zone and is probably generated indirectly by this precipitation and convected equatorward. In the main trough region spread is sometimes present and other times absent. Some of the individual small‐scale irregularities responsible for spread F can extend along magnetic‐field lines for hundreds of kilometers.

Spatial relationship of field‐aligned currents, electron precipitation, and plasma convection in the auroral oval

A technique has been developed that allows the analysis of magnetometer data from Atmosphere Explorer C during those periods when the spacecraft is spinning and eclipsed (not sunlit). The utilization of these magnetometer measurements to determine field‐aligned current structure in conjunction with energetic particle and plasma convection measurements from AE‐C enables us to determine relationships between these phenomena in the nighttime auroral zone. From a study of some 18 auroral oval crossings in the sectors 1800–2200 MLT and 0100–0600 MLT it is observed that, in general, the regions of energetic electron precipitation (&gt;1 keV) span both region 1 and region 2 currents with the poleward boundary of the region 1 currents approximately coincident with the poleward kilovolt electron boundary. Simultaneous observations of plasma convection indicate that the reversal from sunward to antisunward convection is also, in general, collocated to within a degree of invariant latitude to the poleward field‐aligned current boundary in both the evening and morning sectors. It is seen that the low conductivity in the winter polar cap implies the absence of field‐aligned currents in that region. The data indicate that the region 1 currents in the late evening and early morning sectors lie on closed field lines extending earthward from the inner edge of the magnetospheric boundary layer.

The anisotropy of high‐latitude nighttime F region irregularities

The anisotropy of intermediate‐scale, F region irregularities in the nighttime auroral zone is described. The study is based upon spaced‐receiver phase scintillation measurements made with the Wideband satellite at Poker Flat, Alaska. A systematic dependence of irregularity anisotropy with local time and magnetic latitude is observed, suggesting convective control. Sheetlike irregularities are confined to the zone of east‐west drift near the equatorward boundary of the auroral zone, and at the flow reversal, or Harang discontinuity, the cross‐field extension of the sheets is reduced. The extension of rodlike irregularities, which are observed poleward of the zonal convection boundary, also shows apparent convection dominance. Mechanisms for convection control of the anisotropy are discussed.

Morning sector ion precipitation following substorm injections

The auroral ion measurements (0.4–30 keV) made from the polar satellite Aureol 1 have revealed two types of precipitation with different energy characteristics in the auroral and subauroral nighttime zone: (1) In the entire nighttime auroral zone a proton precipitation with an average energy ranging from ≃5 keV to at least 25 keV, which coincides with the region of auroral Balmer series lines emission as observed from the ground (Crasnier et al., 1974). The large scale characteristics of this precipitation reflect the adiabatic acceleration of the protons in the plasma sheet and their drift motions in the outer magnetosphere (Galperin et al., 1976, 1978). (2) A subauroral precipitation of ions with energy less than 12 to 15 keV limited to the morning sector between approximately 0000 and 0500–0600 MLT. This newly observed structure is sporadic and takes place in the hours following the injections of plasma sheet particles into the inner magnetosphere at the onset of magnetospheric substorms. The ionospheric region affected by this precipitation corresponds, in the equatorial plane, to the zone in which Alfvén layers result from the ion drift pattern (from the added effects of convection and corotation electric fields and from magnetic field gradient). It is shown that the electric and magnetic field models derived by McIlwain (1972) from the energy dispersion of the particles injected at 6.6 RE at the substorm onset accounts for the main spatial and energy characteristics of these low‐energy ion subauroral precipitation. This result is obtained by extending the logic used to study the proton ‘noses’ (Smith and Hoffman, 1974; Konradi et al., 1976) from the evening side to the morning side of the inner magnetosphere, and this makes it possible to obtain, from the injection boundary concept, a coherent approach to the substorm injection features at different local time sectors.

The structure of localized nighttime auroral zone scintillation enhancements

Simultaneous observations of the polar‐orbiting Wideband satellite at two auroral zone stations (separated by approximately 200 km along the geomagnetic meridian) were made during November 1977. The latitudinal distribution of the average scintillation occurrence at each station shows a local maximum at the point where the propagation vector lies within an L shell. This supports the hypothesis that the enhancement is a geometrical effect due to L shell aligned, sheetlike irregularities. The detailed structure of the average scintillation occurrence as well as the scintillation patterns on individual passes shows, however, that the source regions are confined in latitude. From a detailed analysis of total electron content data we were able to show that the local scintillation enhancements occur in a region of enhanced F region ionization with a steep gradient at its equatorward edge. The source region lies within the general confines of the diffuse aurora, often with too limited a latitudinal extent to cause an enhancement at both stations. These new results are discussed in light of recent theoretical analysis by Ossakow and Chaturvedi that shows that the current convective instability, which is essentially a stable E×B configuration destabilized by moderate field‐aligned currents, can explain the irregularities. The sheetlike structure is a result of the nonlinear saturation of the instability. The linearly unstable mode, which would give rise to meridional structures, is nonlinearly damped while the dominant nonlinearity favors the perpendicular modes that cause the sheetlike structures.

ISIS 1 observations at the source of auroral kilometric radiation

Observations of auroral kilometric radiation (AKR) are made by ISIS 1 in the source region. The radiation is found to be generated in the extraordinary mode just above the local cut‐off frequency and to emanate nearly perpendicular to the magnetic field. It occurs within local depletions of electron density, where the ratio of plasma frequency to cyclotron frequency is less than 0.2. The density depletion is restricted to altitudes above about 2000 km, and the upper AKR frequency limit corresponds to the extraordinary cutoff frequency at this altitude. AKR is observed from ISIS 1 above the nighttime auroral zone over a wider extent in longitude than in latitude with an intense source region observed most often near 2200 LMT and 70° invariant latitude. It is directly related to inverted V electron precipitation events with an electron‐to‐wave energy conversion efficiency of the order of 0.1 to 1%.

A comparison between N2+4278-Å emission and electron flux in the auroral zone

Visual airglow and low-energy electron experiment data from the Atmosphere Explorer D satellite are used to measure variations in N2(+) density and mixing in the nighttime auroral zone. The intensity of the 4278-A N2(+) emission, proportional to energy flux deposited in N2 by energetic electrons, is determined by a method which corrects for ground reflection affecting the satellite data. Calculations related to electron energy flux are found to show a slight variance from the theoretical predictions of Rees and Luckey (1974). Evidence for high-energy proton precipitation on the southern edge of the aurora and for the presence of parallel electric fields in the altitude range 150 to 800 km is also discussed.

The current systems of the magnetic substorm growth and explosive phases

On the basis of the geomagnetic data of highlatitude arctic stations the development of polar magnetic substorms is examined. It is shown that there exist two current systems of the magnetic substorm: DP 11 and DP 12. 11 is a current system with one westelectrojet in the nighttime auroral zone. That system is peculiar to the break-up phase of a substorm. DP 12 is a two-vortex current system in the polar cap with two auroral electrojets, eastward and westward, of about equal intensity. The DP 12 system is typical for growth and recovery phases. There are two different types of substorm development. The first type is characterized by the DP 12 system during the growth phase. The intensity of this current system increases until the explosive phase begins. The other type does not seem to be characterized by any distinct current system during the growth phase. The commencement of such a substorm is associated with a rapid explosive development of the DP 12 system. A conclusion about the the different origins of the DP 11 and DP 12 current systems is made.