Auroral Features Research Articles

The CDAW-8 substorm event on 28 January 1983: A detailed global study

A small, isolated substorm with an expansion phase onset at 0739 UT on 28 January 1983 was well observed by ground-based instrumentation as well as by low- and high-altitude spacecraft. Because of the comprehensive nature of the data coverage, including ISEE-3 identification of plasmoid signatures in the deep tail (∼ 220 R E) associated with the substorm, we are able to provide a detailed timeline of the growth, expansion, and recovery phases of the substorm. The magnetospheric energy input rates are evaluated using IMP-8 in the upstream solar wind. DE-1 imaging sequences are used to examine auroral features during the growth and expansion phases. Substorm current wedge and expansion onset information is provided by ground-based magnetometer and geostationary orbit (particle and magnetic field) data. The plasma, energetic particle, and field signatures at ISEE-3 are considered within the framework of the near-earth data. Quantitative estimates of substorm energy input and output relationships are made for this case and the timing and physical dimensions of the deep tail disturbance implied are evaluated by the global observations available.

Intensified Charge-Coupled-Device Imager For Auroral Studies

A 100 X100 pixel charge-coupled-device (CCD) area detector combined with an image intensifier, all-sky optics, and a filter wheel has been developed to provide quantitative monochromatic images of the aurora. The design of the instrument provides for a variable exposure rate to accommodate the large dynamic range of auroral intensities, from a minimum of 20 ms up to several minutes. Standard auroral emission features are monitored by four selectable filters, normally at 6300, 5577, 3914, and 4278 A. Depending on the filter utilized, sensitivity can be from 180 to 250 R ADU-1 s-1. The instrument has been used successfully in support of balloon, rocket, and satellite campaigns.

Unusual auroral features observed on January 10‐11, 1983 and their possible relationships to the interplanetary magnetic field

We report unusual auroral features observed by the DMSP‐F6 satellite on January 10 and 11, 1983. These features include the shift of the most active region to the morning sector, the shift of the polar cap center toward the evening sector, a large number of sun‐earth aligned arcs and the oval‐shaped diffuse glow without bright oval arcs. The simultaneous magnetic field observations by the ISEE‐3 and IMP‐8 satellites, together with a simple magnetospheric modeling result, suggest that the unusual features result from both the unusually large By and a large positive Bz component of the IMF.

Origin of the plasma in a cross‐polar cap auroral feature (theta aurora)

Ion composition data obtained from a particularly well observed cross‐polar cap auroral feature (theta aurora) are presented. Two components of the hot plasma are identified, one from the ionosphere below and the second from the distant plasma sheet. These observations provide strong support for the conclusion that the cross‐polar cap auroral feature occurs on field lines closing through the distant plasma sheet or plasma sheet boundary layer. Taken together with the previously published observations, these new results establish substantial constraints on models for the overall polar cap topology. However, they do not unambiguously favor either of the two principal topologies discussed in the literature.

Auroral kilometric radiation/aurora correlation

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all‐sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty‐eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density Ne contours, from the satellite altitude down to the F region ionization peak, were obtained along N‐S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all‐sky camera images. Density cavities (regions where Ne < 100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral‐arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudinal Ne gradients.

Electron precipitation equatorward of the auroral oval and the mantle aurora in the midday sector

In the midday sector, the hard electron precipitation and the associated patchy aurora at geomagnetic latitude ∼65° are the only auroral features (≲ 20 keV) located equatorward of the dayside auroral oval during intense and moderately disturbed geomagnetic conditions. We identify the patchy luminosity in the midday and late morning sectors as the active mantle aurora. The mantle aurora was found by Sanford (1964) using the IGY-IGC auroral patrol spectrographs and which was thought to be non-visual. The precipitating electrons reside mostly at energies greater than several keV with an energy flux of ≳ 0.1 erg cm −2 s −1 sr −1 during geomagnetic active periods. This hard precipitation occurs in a region which is asymmetric in L.T. with respect to the noon meridian. The region extends from the morning sector to only early afternoon (13–14 M.L.T.) along the geomagnetic latitude circle of about 65–70°. The model calculation indicates that the mantle aurora is produced by the precipitation of the energetic electrons which drift azimuthally from the plasma sheet at the midnight sector to the dayside magnetopause during magnetospheric substorms.

Mirror instability and the origin of morningside auroral structure

Auroral optical imagery shows marked differences between auroral features of the evening and morning sectors: The separation between diffuse and discrete auroras in the evening sector is not distinct in the morning sector, which is dominated by auroral patches and multiple banded structures aligned along some direction. Plasma distribution function signatures also show marked differences: downward electron beams and inverted‐V signatures prefer the evening sector, while the electron spectra on the morning sector are similar to the diffuse aurora. We have constructed a theory of morningside auroras consistent with these features. The theory is based on modulation of the growth rates of electron cyclotron waves by the mirror instability, which is in turn driven by inward‐convected ions that have become anisotropic. This modulation produces alternating bands of enhanced and reduced electron precipitation which approximate the observed multiple auroral bands and patches of the morning sector.

Three‐dimensional current flow and particle precipitation in a westward travelling surge (observed during the Barium‐Geos Rocket Experiment)

During the Barium‐GEOS rocket experiment on September 24, 1979 the passage of a westward travelling surge (WTS) was observed over Scandinavia. Extended instrument networks in that area, viz., the Scandinavian Magnetometer Array, the STARE radars, all‐sky cameras, and riometers, allowed simultaneous observation of the two‐dimensional character of magnetic and electric field disturbances and particle precipitation associated with the auroral feature. By combination of the different datasets and additional information from particle and electric field measurements aboard the Barium‐GEOS rocket it was possible to derive the two‐dimensional distribution of ionospheric electric fields and conductivities and to model the three‐dimensional current flow in the vicinity of the westward travelling surge. The main feature of the resulting model current system is the presence of a localized upward field‐aligned current directed out of the head of the surge and fed by a westward electrojet, which is composed of both Hall and Pedersen currents. Secondary ionospheric currents, e.g. a counterclockwise loop of mainly Hall currents around the leading edge of the surge, are found to be responsible for most of the transient effects observed by ground‐based magnetometers as the WTS passed overhead. The most energetic particle precipitation as inferred from cosmic noise absorption measurements and triangulation of auroral arc altitudes is found to be confined to the leading part and central regions of the surge and to travel westward with the visual auroral form.

The instantaneous relationship between polar cap and oval auroras at times of northward interplanetary magnetic field

Optical images of the polar cap region at both 5577 and 3914 Å obtained from 1400 km above the earth have been used to study the relationship between polar cap and oval aurora during periods when the interplanetary magnetic field is strongly northward, i.e., B2 > 3.5 nT. When this rather rare condition occurs, the distinction between the two types of aurora is no longer as clear as depicted on the basis of statistical definitions of the auroral oval. Diffuse, weak emission can fill in the region between the auroral oval and discrete auroral features in the polar cap. The polar cap discrete features can appear very similar to auroral oval arcs in intensity, intensity ratio, and structure. Even more striking are the situations where discrete polar cap features merge with oval auroras. From this study it is concluded that under conditions of large positive B2 the region of closed magnetic field lines can expand poleward to occupy much of the high latitude region.

The auroral 214Å feature

Laboratory data are used to argue that the excitation of the N II doublet near 2145 Å by electron impact on N2 occurs through the ejection of an electron from the 2sσg orbital. The cross section probably exceeds 10−18 cm², as is required if electron impact excitation of N2 is the source of the auroral feature. By extrapolation along the isoelectronic sequence, we estimate a lifetime of 5.8 ms, in harmony with recent ab initio calculations. The auroral observations then imply a rate coefficient for quenching of N+(5S2°) ions in the atmosphere of less than 10−10 cm³ s−1.

Observations of the optical spectrum of the dayside magnetospheric cleft aurora

Optical spectra of the cleft aurora in the region 5000–8500 Å were measured in December, 1977 at Cape Parry, N.W.T. A Michelson interferometer was used at a resolution of 10 cm −1. The auroral features observed were OI (5577, 6300-64, 7774, 8446 Å), OII (7319-30 Å), NI (5200 Å), Hα, O 2 atm (1,1), some weak N 21P bands and possibly some Meinel bands of N 2 +. In addition, nightglow emissions of Na and OH were observed. Theoretical predictions of the OI and NI emission rates using the model of Link et al. (1980) fit the observed rates reasonably well if a 40 eV Maxwellian incident electron spectrum is assumed. The predicted rates for OII exceed the observed value by a factor of 4. It is suggested that the ionization cross-section may be over-estimated.

Particle flux decrease‐increase events at synchronous orbit and the temporal sequence of aurora during substorms

A systematic temporal correlation has been found between the energetic particle intensity variations measured at 6.6 RE and the development of large scale auroral features. The intensification and equatorward drift of eastwest oriented stable discrete homogeneous auroral arc systems coincides with the decrease in energetic particle intensity at 6.6 RE as the nightside magnetosphere develops into a more tail‐like configuration. The subsequent major breakup of the aurora coincides with the recovery in particle intensity as the field returns to a more dipolar configuration. Since this prominent decrease‐increase sequence must be related to the intensification or inward convection of the tail current plasma sheet configuration followed by its diversion or dissipation, the auroral correlation documented here closely links the auroral particle precipitation to the plasma sheet and tail current dynamics.

Electron flow and visual aurora at the Harang discontinuity

We present several examples of the ionospheric electron drift patterns and of the aurora near the Harang discontinuity. The drift patterns were detected with the Scandinavian Twin Auroral Radar Experiment, and the auroral features were determined by using all‐sky camera data. We find the behavior of the discontinuity to be strongly dependent on the level of magnetic activity as measured by the Kp index. During low activity the discontinuity remains at a fairly constant latitude for a considerable length of time. Usually, one to two auroral arcs are observed within the discontinuity. With increasing magnetic activity the Harang discontinuity tends to surge rapidly equatorward with mean speeds ranging from 150 to 600 m/s. This equatorward surge is observed at earlier local times as magnetic activity increases. We have found two cases in which an arc appears in the discontinuity at some point during its equatorward surge. The equatorward surge of the essentially east‐west‐aligned discontinuity appears to be associated with a limited region of strong equatorward electron flow which splits up into eastward and westward flow. This region may form the eastward termination of the discontinuity. The eastward termination of the Harang discontinuity during times of low magnetic activity appears to consist of an equatorward extension of the region of eastward electron flow within a spatially limited region near local magnetic midnight. We find the equatorward surges to be terminated by the sudden activation of an arc in the discontinuity, followed by a poleward expansion. Following these substorms the Harang discontinuity disappears from the field of view. At times, we have observed arcs to brighten and surge poleward in the region equatorward of the discontinuity. The discontinuity did not appear to be strongly affected by these substorms. We find in general that auroral arcs are located in or poleward of the discontinuity, even though active arcs at times appear south of the discontinuity. Diffuse aurora have been observed only equatorward of the discontinuity.

Relationship of noise in the frequency range 100 < f < 500 kHz to auroral arcs and field‐aligned current and implications regarding acceleration of auroral electrons

The magnetic signatures of field‐aligned and ionospheric currents can be used to determine the geometry and strength of the electrical current systems which couple the ionosphere to the outer regions of the earth's magnetosphere. It is now recognized that the currents flowing in the evening sector are an important clue to the understanding of processes leading to explosive substorm instabilities and episodes of intense ionospheric disruption. In this report we have used a combination of ground‐based magnetometer data and Isis 2 satellite data in the form of energetic particles, auroral luminosity, and electromagnetic and/or electrostatic noise measurements to provide a comprehensive picture of processes which are taking place on field lines penetrating the auroral oval in the evening and midnight sectors. We shall show that noise measurements in the frequency range 100 < f < 500 kHz are characteristic of regions of upward current flow which are found in the region of discrete auroral features. We find that the higher‐energy electrons (a few keV) are associated with the higher‐frequency noise. In addition, the highest noise frequencies are found in the center of discrete auroral arc regions with the frequency decreasing on either side of the center of the arc region. We believe that the source of noise in this frequency range is associated with electron beams in discrete arcs whose velocity space distribution contains a region of positive slope. We shall finally discuss the physics of the relationship among the various electromagnetic and energetic particle signatures presented in this study.

The midday, discrete auroral gap

A gap in the midday sector of the auroral oval is examined on the basis of Defense Meteorological Satellite Program (DMSP) satellite auroral photographs. All DMSP photographs recorded in June 1975 over the southern hemisphere were examined, and 38 photographs were found suitable to specify details of auroral features in the midday sector. There is not a single example showing the discrete auroral arcs extending continuously across the noon sector, bridging the morning and afternoon sectors of the auroral oval. Hence a gap, manifested by the absence of extended, discrete auroral arcs, exists in the midday sector of the auroral oval. The gap appears typically to extend over more than 15° of longitude or, correspondingly, more than 1 hour of local time and is centered about the 1000 corrected geomagnetic local time meridian. During an early phase of substorms the gap appears to fill with rays and fragmented arcs and to empty after substorm recovery. Similarly, polar cap arcs appear and disappear at the same times that the gap fills and empties of rays and fragmented arcs. The gap therefore appears to be a semi‐permanent feature of the midday auroral distribution.

On current continuity at the Harang discontinuity

Although the Harang discontinuity has so far been identified in terms of various phenomena (such as ground magnetic fields, ionospheric currents, auroral features, and electric fields), the loci defined by those different phenomena do not always coincide. It is suggested that the Harang discontinuity may not be a line boundary across which the electric field changes its direction simply from poleward to equatorward, but that the field gradually rotates counterclockwise in a narrow region; thus the westward electric field dominates there. In such a case, no field-aligned current is necessarily required to flow from or into the discontinuity region. This view may be contrasted with the conventional view that an intense upward field-aligned current should flow from the Harang discontinuity. A model is presented in which the poleward ionospheric current (the Hall current resulting from the westward electric field) in the Harang discontinuity region connects the eastward electrojet and the westward electrojet.

Photometric investigation of precipitating particle dynamics.

The theoretical basis and the experimental evidence for the interpretation of photometric data in terms of precipitating particle morphology will be reviewed. Particular emphasis will be laid on the interpretation of auroral morphology in terms of the spectroscopic signature of the precipitating particles. The meridian scanning photometric data will be discussed briefly to emphasize the auroral morphology related findings. The new type of monochromatic TV system which was specially developed for two-dimensional spectrophotometric auroral imaging will be described. The auroral morphology related findings will be discussed according to the local time sector of the observations. The early evening auroras are characterized by soft electron precipitation on the equatorward edge of the auroral oval. This precipitation is not a steady diffuse region but it is structured and the structures show coherent convective motion often in the westward direction. Harder precipitating features superimpose on this soft precipitation. These harder features (electrons 2keV or larger) are observable by all-sky cameras and they participate in the substorm auroral dynamics. Following substorm onsets and northward expansions the electron auroras harden considerably and the protons expand poleward. During early morning the diffuse soft regions occur poleward of the hard auroral features. These early morning hard auroral features often show very hard pulsating patches.

Dependence of substorm occurrence probability on the interplanetary magnetic field and on the size of the auroral oval

The dependence of substorm occurrence probability on the north-south component Bz of the interplanetary magnetic field (IMF) and on the size of the auroral oval is examined on the basis of two independent data sets (Isis 1 and 2 low-energy electron data and all-sky camera data from the Alaska meridian). The occurrence of substorms is identified by the electron precipitation pattern (Isis data), the auroral features in the Alaskan sector, and available ground magnetic records. The substorm occurrence frequency increases as the oval expands and as the Bz component of the IMF decreases. It increases from approximately 25% to 100% for Bz values of the IMF ranging from +7 to −5 nT; here Bz values are averaged for 1 hour preceding the time of satellite passage and the onset of substorms observed by the all-sky cameras. The occurrence probability of quiet times increases with an increasing value of the northward IMF. It is interesting to note that there are almost no periods without substorms when the IMF has a large southward component, but substorms do occur even when the IMF has a large northward component. Since there is a close relationship between the direction of the IMF and the size of the auroral oval, our finding that the occurrence frequency of substorms increases with the expansion of the auroral oval suggests that the substorm probability may be related to the amount of energy stored in the magnetotail.

The spatial relationship of field-aligned currents and auroral electrojets to the distribution of nightside auroras

In this paper, nearly simultaneous sets of ground-based magnetometer data, magnetic perturbations recorded at 800-km altitude by the polar-orbiting Triad satellite and auroral imagery and information on precipitating electrons in the energy range 200 eV < E < 20 keV obtained from the DMSP satellites are analyzed for intervals of moderate magnetospheric activity during which substorms were in progress. It is found that the boundaries of intense (>2 kR at 5577 Å) auroral features coincide with the boundaries of field-aligned current regions defined by the Triad data. In the morning sector the downward field-aligned current flows in the poleward portion of the auroral oval, and the intensity of this current appears not to be a function of the intensity of the related auroral features. These results suggest that the current is carried by upgoing thermal electrons from the ionosphere. On the equatorward side of the oval the region of the upward field-aligned current coincides with the bright aurora regime and is probably carried by precipitating energetic electrons. In the evening sector, regions of intense upward field-aligned currents are found to coincide with discrete auroral arcs and fluxes of precipitating electrons with energies peaked at several keV in the region immediately poleward of the eastward electrojet. This region is thought to coincide with the extension of the westward electrojet from the morning sector, and precipitating electrons in the bright auroral forms are thought to carry the upward field-aligned current in this region. In a narrow latitudinal area poleward of the upward current where no auroral luminosity is detected, downward field-aligned current is often observed, indicating the importance of ionospheric thermal electrons carrying the downward current. This current is thought to be connected to the equatorward Pedersen current of the equatorward electric field that is dominant in the westward electrojet region. In the equatorward half of the evening auroral oval the diffuse aurora is located in the region of the downward field-aligned current and, in general, collocated with the eastward electrojet.

Instrument for the monochromatic observation of all sky auroral images

To investigate the dynamics of auroras and faint upper atmospheric emissions, a new type of imaging instrument was developed. The instrument is a wide field of view, narrow-spectral-band imaging system using an intensified S.E.C. TV camera in a time exposure mode. Pictures were taken at very low light levels of a few photons per exposure per resolution element. These pictures are displayed in the form of a pseudocolor presentation in which the color represents spectral ratios of two of the observed auroral spectral emission features. The spectral ratios play an important part in the interpretation of auroral particle dynamics. A digital picture processing facility is also part of the system which enables the digital manppulation of the pictures at standard TV rates. As an example, hydrogen auroras can be displayed having been corrected for nonspectral background by subtracting a picture obtained by a suitable background filter. The instrumentation was calibrated in the laboratory and was used in several field xperiments. Elaborate exposure sequences were developed to extend the dynamic range and to cover the large range of auroral brightnesses in a fairly linear manner.