Characteristics of polar cap aurora

The characteristics of extremely high-latitude dayside auroras are examined by using auroral TV data obtained at Godhavn, Greenland, and simultaneous DMSP particle data. Two different kinds of aurora are found near the pre-noon sector, namely (1) the polar arc: this aurora is observed during quiet periods and originates from the dayside region. It is related to about 100 eV electron precipitation or less, and (2) the polar corona: this aurora is observed during disturbed periods and the appearence latitute of this aurora is confined within a certain region about 70–80° MLAT. It is related to a few hundred eV electrons. These results suggest that the origin of the polar arc seems to be the plasma mantle or low-latitude boundary layer, and the origin of the polar corona seems to be the low-latitude boundary layer or Boundary Plasma Sheet.     

Photometric observations of proton aurora at Sanae

Proton aurora was monitored at Sanae (invariant latitude 60°S) by recording Hβ emission with a tilting-filter photometer, and observed for K_p⩾ a 3_. Study of two nights of intense auroral activity during 1978 revealed that the times of onset of proton and electron auroral substorms and magnetic substorms agreed within 5 min. Further, electron aurora occurred poleward or equatorward of proton aurora for a location under the eastward or westward auroral electrojet respectively.     

Scintillation at high latitudes during winter magnetic storms

Scintillation observations are described which were made at Kiruna in northern Sweden during three magnetic storm periods in the winter of 1984–1985. The results were obtained using transmissions from the multisatellite NNSS system, so that it has been possible to chart the development of scintillation activity over some 20° of geomagnetic latitude as a function of time for several days throughout each storm. A region of strong scintillation at the highest latitudes near magnetic noon is a common feature on all but the quietest days. This feature, probably associated with soft particle precipitation into the cusp, shows an abrupt boundary which moves equatorwards as the disturbance develops. In the magnetic midnight sector two latitudinally separate zones of scintillation are found, patchy at high latitudes although more sustained in the auroral zone. An absence of auroral scintillations around midnight UT can be followed by prolonged intense scintillation activity at auroral latitudes during the early morning hours on some disturbed days.     

Storm after-effects observed at Ushuaia and Kerguelen

D-region disturbances have been detected at mid-latitudes after intense magnetic storms by means of a wide range of radiowave signals. Two different mechanisms have been suggested to account for these storm ‘after-effects’: one implies precipitation of energetic electrons from the radiation belt; the other, transport of neutral constituents from the auroral zones.This paper presents observations of abnormal enhancements of ionospheric absorption arising after two major magnetic storms occurring in March and April 1976. The measurements were made at Ushuaia (54.8°S, L = 1.7) and at Kerguelen (49.4°S, L = 3.7). The former were obtained by means of the pulse reflection method (A1) at MF and the latter by the riometer technique. It is shown that electron precipitation can explain the effects observed at Kerguelen but not those at Ushuaia which also depart significantly from the ‘winter anomaly’ trend observed at that site. The abnormal ionization at Ushuaia is attributed to transport from the southern auroral zone.     

Recurrent propagating auroral forms in pulsating auroras

Observations of pulsating auroras with an all-sky Isocon TV system made from Poker Flat Research Range, Alaska, and Andøya rocket range, Norway, have revealed that auroral forms propagating like waves, (here termed recurrent propagating auroral forms) predominantly in the meridional direction, are a significant proportion of the group of auroras commonly labelled pulsating auroras. The wavelike forms are described and compared with pulsating patches. Characteristics of the two types differ sufficiently, that the possibility of different mechanisms should be considered. Propagation of the wave forms continues up to discrete arcs on the poleward edge of the diffuse aurora. One example is presented where no enhancement was observed within black aurora when the wave forms encountered this on their propagation path.     

Optical measurements of a nightside poleward expanding aurora

Coordinated optical observations were performed from the poleward side of the midnight auroral oval. Height measurements of the auroral emissions at 4278, 5577 and 6300 Å, as well as their intensity ratios in the poleward expanded auroral substorm, have been carried out. The findings indicate a significantly softened electron spectrum compared with similar data from the equatorward part of this substorm. Typical values for the poleward expanded aurora are 300 eV and lower, while keV particles dominate the auroras at 10° lower latitudes. Emission altitudes and spectral characteristics are comparable to the transient burst emissions frequently observed from the same site in the post-noon sector, i.e. within the cusp.The 6300 Å atomic oxygen emission is used as a tracer of F-region wind and temperature. Interferometer observations show that there exists a prevailing crosspolar antisunward wind, increasing with geomagnetic activity to several hundred m s⁻¹. The temperature shows an increase of 150 K associated with high geomagnetic activity.     

Sondrestrom and EISCAT radar observations of poleward-moving auroral forms

During many magnetospheric substorms, the auroral oval near midnight is observed to expand poleward in association with strong negative perturbations measured by local ground magnetometers. We show Sondrestrom and EISCAT incoherent scatter radar measurements during three such events. In each of the events, enhanced ionization produced by the precipitation moved northward by several degrees of latitude within 10–20 min. The electric fields measured during the three events were significantly different. In one event the electric field was southward everywhere within the precipitation region. In the other two events a reversal in the meridional component of the field was observed. In one case the reversal occurred within the precipitation region, while in the other case the reversal was at the poleward boundary of the precipitation. The westward electrojet that produces the negative H-perturbation in the ground magnetic field has Hall and Pedersen components to varying degrees. In one case the Hall component was eastward and the Pedersen component was westward, but the net magnetic H-deflection on the ground was negative. Simultaneous EISCAT measurements made near the dawn meridian during one of the events show that the polar cap boundary moved northward at the same time as the aurora expanded northward at Sondrestrom. Most of the differences in the electrodynamic configuration in the three events can be accounted for in terms of the location at which the measurements were made relative to the center of the auroral bulge.     

Oscillations in D-region absorption at periods of one to two months

One to two month oscillations in D-region absorption are found in seven years of daily ƒ-min data from low latitude stations at Singapore (1°N, 104°E) and Rarotonga (21°S, 160°W). Coherency (cross-spectral) analyses reveal that solar flux variations account for much of the ƒ-min variance at these periods. Over the range of periods from 10 to 200 days, statistically significant linear correlation is found between the ƒ-min time series and contemporaneous 10.7cm solar flux measurements at periods of 16–19 days, the 26–29 day solar rotation band, and a broad band covering 43–80 day periods.     

The auroral distribution and its mapping according to substorm phase

An attempt is made to reconcile two competing views as to where the auroral distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a near-Earth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic 1–10 keV electrons and 0.1–1 keV structured electrons. The ionospheric local time sector between 13 and 18 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an auroral distribution specific to the substorm recovery phase. These auroral observations provide a means of inferring important information concerning magnetospheric topology.     

Low latitude signatures of substorm activity

Although magnetospheric substorms have their most easily detectable observable consequences in the high latitude auroral oval, most of the early studies of the phenomenon took place using data from low latitude observing sites. Many of these early studies concentrated on magnetic storms, yielding observations and interpretations of the ring current and sudden storm commencements which are valid to this day. Over the years, ground-based observations have been concentrated in the high latitude regions under and immediately adjacent to the auroral oval and studies of storm and substorm phenomena using low latitude data have been few in number. Despite the intensity of research activity in recent years, the physics of the substorm process still remains a matter of controversy. The STEP period represents an excellent opportunity for substorm physicists to solve some of the outstanding problems they still face. Space scientists monitoring the geospace environment at observing sites well equatorward of the auroral oval have an excellent opportunity to contribute to the ultimate solution of the substorm problem during the STEP interval, however they will have to focus on specific observational manifestations of substorms if they are to optimize their probability of success. In this paper I shall try to point out some areas where successful observations and interpretation of substorm phenomena at low latitudes would be extremely useful in helping the substorm community resolve their outstanding scientific problems.     

Variations in Mid‐Latitude Auroral Activity During the Holocene*

Estimates of the past location of the auroral zone can aid archaeologists in the interpretation of documented light phenomena in the night sky; in particular, during periods without written records and in regions where aurorae are scarce today. Aurora occurrence largely depends on solar activity as well as the tilt and strength of the geomagnetic field. Here, we introduce a tool (AUREST) that visually combines the time‐dependent variations of these parameters to provide relative estimates of aurora occurrence in mid‐latitude regions of the Northern Hemisphere over the past 10 000 years. Tests against historical aurora observations show good compatibility with AUREST.     

Neutral atom precipitation—a review

The production of energetic neutral atoms by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted many years ago, and there are now a number of measurements of the effect of the impact of these energetic atoms on the thermosphere. Theoretical models of the process have been developed. The latitude variation of the precipitating flux depends very much on the pitch angle distribution of the ions in the ring current, and on the L shell on which they are located.The production of a belt of trapped particles at low altitude near the magnetic equator may occur when neutral atoms re-ionize and become trapped on impacting the thermosphere, and this belt has been found in particle measurements near the equator and is enhanced during periods of magnetic activity.A region of enhanced optical emission due to precipitating neutrals is found in the thermosphere near the magnetic equator in both disturbed and quiet times, implying a low L value and/or pancake pitch angle distribution for the ring current particles that give rise to these neutrals. An isotropic pitch angle distribution is present in parts of the ring current at time during magnetic storms. This gives rise to neutral atom precipitation at all latitudes, and particularly of particles near 90° pitch angle in the region of SAR arc occurrence, about 10° in dip latitude equatorward of the isotropic region.The rate of energy deposition and the rate of production of ionization in the thermosphere depend on the ion species present in the ring current; their energy spectra, and on the distributions of the ions with L value and pitch angle. The rate of energy deposition may at times reach 10⁻² to 10⁻¹ mWm⁻², sufficient for significant heating and wind generation. The rate of production of ionization in the thermosphere at night may be much greater than that of other low latitude night-time ionization sources.     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

The response of geomagnetic spectral composition to solar wind during storm time

In this paper 16 geomagnetic storms in 1968–1978 recorded at 8 magnetic observatories located from polar to equatorial regions in the λ= 120°E longitudinal zone and its vicinity have been analysed. The horizontal component H traces of 27 h intervals have been sampled once every 1.5 min. The time sequences of the data thus obtained have been processed by the method of digital filtering and maximum entropy spectral analysis (MESA).The results of the analysis are compared with the associated solar wind parameters. It confirms that the geomagnetic disturbances are controlled by the solar wind in several ways, i.e. geomagnetic disturbances respond differently to various solar wind parameters or to the different ranges of them. The north-south component of the interplanetary magnetic field (IMF) B_z., the IMF latitude θ and the solar wind velocity V play the most important part in inducing geomagnetic storms.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Behaviour of HILAT scintillation over spitsbergen

Results of the amplitude scintillation morphology of the HILAT satellite 137 MHz beacon transmission as measured at the Polish Polar Station at Hornsund, Spitsbergen (Δ = 73.4°) are presented. Seasonal, diurnal and latitudinal dependencies of scintillation intensity on magnetic activity were analyzed from over 2250 satellite passes recorded at solar minimum between April 1985 and March 1986. Regions with strong scintillation intensity appear to follow the auroral oval expansion and to move sunward with increasing level of magnetic activity. Maximum amplitude scintillation region coincides with the dayside cusp/cleft position during high magnetic activity. The dawn-dusk asymmetry in scintillation intensity is more distinct in winter than other months. The estimated summer/winter ratio of scintillation intensity is 1.4: 1. Numerical simulations compared with the observational results indicate that high latitude irregularities < 1 km are field-aligned and rod-like rather than sheet-like.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

Neutral density cells in the high latitude thermosphere—1. Solar maximum cell morphology and data analysis

A new high latitude thermospheric neutral density structure has been revealed in NCAR-TIGCM simulations at 120–350 km altitude. The structure consists of density cells above 50° latitude with radii of approximately 1000 km. There are between two to four cells present depending on the altitude and magnetic activity. For example, at 200 km under magnetically active conditions, the density structure consists of four cells: low density cells are located near dawn and dusk and high density cells are located near noon and midnight. Density variations among the cells range from 5 to 50% for magnetically quiet and active conditions respectively. The cells are present at all seasons, for a wide range of magnetic activity levels, and at solar minimum and solar maximum. The density cell morphology is established for equinox solar maximum as a function of altitude and magnetic activity. Departures of the cell structure from this morphology due to seasonal and solar cycles are discussed. The cell morphology provides a new framework in which to interpret lower thermospheric density data. Data to test and confirm the model predictions were provided by the SETA-1 satellite.     

Simultaneous ground-based observations of polar cap arcs and spacecraft measurements of particle precipitation

In order to investigate the particles which produce the polar cap aurora at the Vostok station in Antarctica, charged particle data obtained by the DMSP satellites for some days in a period from April to August 1985 were surveyed. Due to the satellite orbit the local time range in which the data were available was the morning sector. For all the events when sun-aligned arcs were observed on the ground the simultaneous DMSP measurements on almost the same field line showed an increased integral number flux J. > 10⁸ (cm⁸/s/sr)⁻¹ of the precipitating electrons with energy E_e > 200 eV. The electron spectra with double peaks are typical of intense electron precipitation in the polar cap arcs. The most noticeable feature of ion spectra in the polar cap arcs is the prominent minimum in ion flux in the energy range 0.1 < E_i < 1 keV in contrast with the oval precipitation ; this feature gives the possibility to separate the polar arcs from the aurora in the oval. In some events the satellite crossed the system of two widely separated arcs ; one of them was a sun-aligned arc whereas the other was circular at constant latitude according to the Vostok data. The analysis of the DMSP electron and ion precipitation data has shown that in these events the latitude-oriented arcs are located in the polar cap and not in the auroral oval.     

Observations of thermospheric neutral winds by the UCL Doppler imaging system at Kiruna in northern Scandinavia

Since the 1982/1983 winter, the UCL group, in collaboration with the Swedish Institute for Space Physics (previously Kiruna Geophysical Institute), has operated a Doppler imaging system at the high latitude station of Kiruna (67°N, 22°E). The Doppler imaging system is an imaging Fabry-Perot interferometer of 13.2 cm aperture. This instrument has been operated on a ‘campaign’ basis for mapping thermospheric winds using the OI emission at 630 nm (240 km altitude) from a region up to about 400 km radius about Kiruna. In November 1986, the performance of this wide-field Doppler imaging system was augmented by improvements to the detector and all-sky optics. We present data from December 1986, obtained during periods with both clear skies and active auroral and geomagnetic conditions. Maps of the neutral wind flow within the auororal oval during disturbed conditions and near magnetic midnight show continuous and rapid changes of thermospheric winds. The typical scale sizes of eddies observed within the mean flow around magnetic midnight are 100–300 km, with fluctuations at all time scales resolved by the 10 min between successive Doppler images. The local and short period fluctuations appear to be a filtered response of the thermosphere to rapid local variations of the convection and precipitation patterns, within a background of global scale changes