C.D. Anger

A uniform belt of diffuse auroral emission seen by the ISIS-2 scanning photometer

Abstract One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times. The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (Kp = 1−3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 A are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm. It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.

The diffuse aurora

Abstract A study of ground-based all-sky photographs substantiates the presence of the diffuse auroral belt as seen by the ISIS-2 (polar orbiting satellite) scanning auroral photometer. The intensity of the diffuse aurora increases when discrete auroras become active; in particular the diffuse aurora is most clearly seen equatorward of westward travelling surges. However, in the morning sector, it may or may not be detectable near eastward drifting patches in all-sky photographs. Some of what has been previously identified visually and in all-sky photographs as the proton aurora probably is a part of what we identify here as the diffuse aurora. The diffuse aurora appears sometimes to branch out into two, one along the auroral oval and the other along a constant geomagnetic latitude circle. The latter probably corresponds to the mantle aurora and the drizzle zone precipitation.

A global view at the polar region on 18 December 1971

Abstract The ISIS-2 scanning auroral photometer surveyed the polar region during three successive passes on 18 December 1971, at times when Kp values were still high due to an intense magnetic storm which began on 16 December. Two very bright (IBC III) auroral substorm patterns were seen to correspond to rather weak magnetic substorms (about 300 γ in magnitude). A large spiral auroral pattern, with intensity of the order of 100 kR and a size of about 1300 km, was present in the polar cap; it gradually decreased in size and intensity during the interval 0200–0600 UT. A region of enhanced 3914 emission was present in the noon sector of the auroral oval between 0200 and 0400. The presence of the diffuse auroral belt is also evident at all local times during this period, extending down to about 61° corrected geomagnetic latitude in the midnight sector.

An observation of polar aurora and airglow from the ISIS-II spacecraft

Abstract This is a preliminary but comprehensive report on coordinated data obtained with the ISIS-II spacecraft, fourth in the ISIS series, launched 1 April 1971, into a near circular 1400 km orbit. The capabilites of the ISIS-I spacecraft have been extended in a number of ways, including the global mapping of the 3914 A, 5577 A and 6300 A emissions. Data obtained during a 30-min pass over the south pole depict the nightside oval and polar cap, as well as mid-latitude airglow effects; these data are described and discussed.

Topside optical view of the dayside cleft aurora

Abstract Photometers on the ISIS-II spacecraft provide a view of the atomic oxygen 5577 and 6300 A emissions and the N 2 + 3914 A emission detected as dayside aurora in the magnetospheric cleft region. The 6300 A emission forms a continuous and permanent band across the noon sector, at about 78° invariant latitude, with a defined region of maximum intensity that is never less than 2kR (uncorrected for albedo), and is centred near magnetic noon. There are significant differences in the intensity patterns on either side of noon and their responses to geomagnetic activity. Discrete 3914 A auroral forms appear within this region, at preferred locations that cannot be precisely specified, but which tend to the poleward edge of the 6300 A emission in the evening, and the equatorward edge in the morning where the difference between the two emissions is greatest. It is concluded that the discrete auroras observed by all-sky cameras in the day sector do follow the 6300 A emission through the cleft region, though a definite cleft boundary is not defined. Substantial 6300 A emission having a peak intensity near noon is also seen in the low latitude “outer auroral belt”, while the diffuse 3914 A emission tends to show a relative minimum near noon. On the morning side the 3914 A intensity is displaced to lower latitude and earlier local times, compared to the 6300 A emission.

A Monte Carlo analysis of the passage of auroral X-rays through the atmosphere

Abstract The deep penetration of auroral X-rays into the atmosphere has been examined by means of a Monte Carlo calculation in order to determine the effects of atmospheric scattering and absorption on spectra observed at balloon altitudes. Compton and coherent scattering and photoelectric absorption are taken into account in the calculation. The results show that exponential type X-ray energy spectra steepen on penetration through the atmosphere in the energy range 50 to 200 keV due to the fact that the majority of X-rays reaching typical balloon altitudes between 5 and 20 g/cm 2 have suffered energy degradation from Compton scattering. The slope of the spectrum indicated by a collimated detector is found to be more dependent on source geometry and detector source distance than it is for an omnidirectional detector. In fact, the latter detector causes almost constant and hence predictable spectral distortion, irrespective of source geometry or detector-source distance. Data are provided to allow the reader to deduce the parent X-ray spectrum and flux from balloon measurements for different atmospheric depths, detectors, source geometries and energy spectra.

Morphology of electron precipitation during auroral substorms

Abstract A Study of auroral substorms using coordinated measurements of a number of parameters at Ft. Churchill, Manitoba (L = 8) in October 1963 has yielded interesting conclusions on the electron precipitation close to the northern auroral boundary, the morphology of which appears to follow closely the working model proposed by Akasofu. Extensions to this model are suggested that include high energy electron precipitation (>30 keV) as determined from X-ray measurements at balloon altitudes. During surges and auroral substorms near local midnight, very localized precipitation of electrons with a wide spectrum of energies produced a sharp northern optical and radio absorption border which moved rapidly northwards and contained the auroral electrojet. During the decay phases of the substorms, a region of high energy precipitation receded southwards in advance of the northern border of luminous aurora and appeared to be accompanied by the ionospheric electric current. In contrast to this, the optical northern border produced no noticeable auroral absorption or magnetic activity on its southward overhead passage later in the substorm. Systematic spectral changes near the border indicated a softening of the electron spectrum above 40 keV with increasing northward position (or increasing L value), in agreement with satellite observations. Highly structured enhancements of precipitation of high energy electrons occurred at times when small scale folds in auroral bands were reported during break-up and at times of occurrence of Type B aurora.

Polar cap optical aurora seen from ISIS-2

Although the optical aurora represents one of the least direct ways of observing magnetospheric particles and fields it is nevertheless one of the most powerful. The polar atmosphere acts as a giant scintillator, reproducing by virtue of its connection to magnetospheric field lines the complex fields, patterns, and motions of the magnetospheric plasma. Remote sensing of these optical emissions from a high altitude satellite can provide a detailed snapshot of the magnetosphere as it projects into the polar atmosphere.

Intensity ratio I(5577)/I(3914) in Type B red aurora

Abstract Ground-based observations of Type B aurora associated with a westward travelling surge indicate that decreases of up to 40% in the ratio of I(5577)/I(3914) occurred at the region of peak luminosity when compared to parts of the surge removed from the Type B aurora, and to a homogeneous arc at a similar elevation angle. Also, a sharper cutoff was observed for the 5577 A luminosity profile at the lower edge of the Type B aurora. Both effects imply increased quenching or decreased excitation of the 5577 A emission, consistent with a lower altitude for the Type B aurora. The diffuse glow equatorward of the surge showed only a slight decrease in ratio at the time of occurrence of the Type B aurora. The characteristics of Type B aurora are highly suggestive of a local mechanism for particle acceleration.

Auroral colour variations

Abstract Recently overshadowed by instrumental work, visual observation and colour vision theory can nevertheless provide useful information and tests of auroral theories, at least for bright displays. Colour calculations based on a normal auroral spectrum agree with its visual appearance, but Type B displays require drastic changes in red/green emission ratios around 85 km. These changes are consistent with direct electron excitation of atomic oxygen, whose relative concentration becomes negligible below 85 km. The cause of the red lower border is more likely to be N2 1PG emissions rather than O2+ 1NG. The N2 1PG intensity is at least 360 kR, corresponding to a minimum energy flux of 7 × 1013 eV cm−2sec−1 of electrons with energies above 50 keV. Particle and X-ray observations of similar very large fluxes are infrequent, consistent with the rarity of Type B displays. Visibility of Type A red aurora requires a minimum 6300 A intensity of 65 kR, at altitudes where quenching is not important. Blue sunlit aurora implies an N2+ 1NG intensity of probably 500 kR. The time constants of prominent auroral emissions account for such transient effects as coloured edges in moving auroras, brief flashes of green and abrupt changes from green to red. Together with steady state effects, these appear to explain most descriptions in the literature of auroral colours.