Substorm Injection Research Articles

Downward flowing ions and evidence for injection of ionospheric ions into the plasma sheet

Energetic (keV) protons and 0+ ions with field aligned pitch angle distributions and peak fluxes up to ∼ 7 × 107 (cm² s sr keV)−1 have been observed streaming downward in the high altitude (∼ 1 RE) auroral ionosphere. In two examples described in detail, the correlations observed between the downward flowing and coincident trapped ion populations suggest a common origin. Downward flowing ion events are a much less frequent phenomenon than upward flowing ion events in the auroral region. The significant difference implies that the energetic upward flowing ions from the auroral ionosphere are commonly injected into the trapped population of the plasma sheet, and furthermore that parallel electric fields involving potential drops of ≳500 V are directed preferentially upward in the altitude range from 2000 km to ∼ 3 RE. Spatially localized regions of enhanced hot (∼keV) plasma density were frequently observed in the low L portion of the plasma sheet. The statistical location of these ‘plasma clouds’ correlates well with the substorm injection boundary near dusk inferred by McIlwain (1974) and with the upward flowing ion events observed at ∼ 1 RE altitude (Ghielmetti et al., 1978a, b). The downward flowing ion events occurred preferentially within such ‘plasma clouds.’ The results described in this paper suggest that upward flowing ions from the auroral acceleration regions are responsible for both the downward flowing ions and for at least some of the plasma clouds. A significant fraction of the equatorial substorm injected plasma clouds may thus result from this ionospheric source region.

Pc 3 magnetic pulsations and precipitation of energetic electrons

Data from the synchronous altitude satellite ATS 1 and near‐conjugate measurements of bremsstrahlung X rays and ground magnetic variations have been used to analyze an event of modulated auroral zone electron precipitation and magnetic pulsations in the Pc 3 range. Transverse, azimuthal, nearly linearly polarized waves, strongly peaked at ∼25‐s period were observed at ATS 1 from 0600 to 1000 LT on August 18, 1967, diminishing in intensity thereafter. Weak pulsations with periods ≲18 s and wave properties similar to the above followed in the interval from 1300 to 1700 LT. Ground magnetic pulsations at College, Alaska, were observed mainly in the transverse H and D components with maximum power spectral density in the period range 25–40 s prior to 1300 LT and from 20 to 30 s thereafter. Four main intervals of modulated electron precipitation, centered roughly on local magnetic noon, were noted in the X ray data obtained near Fort Yukon, Alaska. The intervals, each lasting for ∼30 min, were separated by ∼90 min. Fluctuations of the 50‐ to 150‐keV trapped electrons at ATS 1 during the precipitation event remained typically within 1 or 2σ of the average counting rates and showed no clear association with pulsations in either the X ray or the magnetic data. The origin of the Pc 3 waves is attributed to local field line resonances induced by Kelvin‐Helmholtz instability at the magnetopause. The observed wave periods can be accounted for by a field line distribution of plasma density of the form r−4 with equatorial values at r = 6.6 RE of order 1 cm−3. The wave resonance model can satisfactorily explain observed differences in the pulsation activity at the ground, balloon, and satellite if account is taken of the spatial sensitivities of the different techniques and the location of observing sites with respect to the probable location of resonant field lines. Although the event pertains to a disturbed period, there is insufficient evidence to associate gross temporal changes in the intensity of pulsation activity with the occurrence of specific substorms. However, the data suggest that electron precipitation pulsations will be found to correlate with Pc 3 magnetic pulsations when substorm injections coupled with azimuthal drift provide enhanced energetic particle fluxes within dayside resonance regions.

The relationship between the harang discontinuity and the substorm injection boundary

A correlative study of characteristic features observed in the ATS-5 particle data, in the Chatanika electric field and ionospheric conductivity data has been performed. It is found that distinct variations in the electric field are observed at Chatanika at the onset of a precipitation event at geostationary orbit. This is probably the effect of the transient electric field inferred by McIlwain (1973). A turn in the meridional component from north to south is observed at Chatanika at about the same local time as the ATS-5 satellite is crossing the injection boundary. This turn in the electric field at Chatanika which is also related to strong particle precipitation is probably due to the crossing of the Harang discontinuity.

Monochromatic all-sky observations and auroral precipitation patterns

An all-sky imaging system capable of fast monochromatic imaging at very low light levels is described; the system allows real time display of regions of proton precipitation and of soft and hard electron precipitation in the auroral zone. Sample data illustrate various auroral types previously classified from photometric data and/or from particle data from satellites. Time-lapse film throughout an entire night shows the position and intensity of plasma sheet precipitation, superposition of a harder electron aurora, the relative positions of proton aurora and diffuse aurora, and the changing character of the aurora as a function of local time and substorm activity. These patterns are discussed in terms of the substorm injection boundary model (McIlwain, 1974).

Plasma Injection at Synchronous Orbit and Spatial and Temporal Auroral Morphology

Analysis of ATS 5 particle spectrograms and simultaneous meridian-scanning photometer data obtained at the base of the ATS field line has revealed systematic relationships between temporal and spatial auroral morphology and particle-injection events. The details of these relationships depend on previous injection events, the local time of the observatory with respect to current-injection events, and the extent and magnitude of current injections. A persistent zone of weak aurora is related to steady plasma-sheet drizzle, and the equatorward edge of this aurora delineates field lines threading the inner edge of the plasma sheet. Auroras associated with injection events superimpose on this preexisting pattern, with the lowest latitude of new precipitation depending on local time. In general, plasma energization is not required between the equatorial plane and the ionosphere to account for observed auroral intensities. The data indicate occasional occurrence of enhanced loss-cone fluxes, suggesting low-altitude field-aligned acceleration. A summary figure shows how various characteristic auroral-precipitation regions are related to the shape, extent, and orientation of the substorm injection boundary and to the subsequent drift paths of injected plasma.

Observation of the plasma sheet during a contracted oval substorm in a prolonged quiet period

This report presents an interesting observation of a substorm which occurred on a contracted oval, with simultaneous particle measurements by Imp 6 (r ∼ 11 RE) and ATS 5 (r ∼ 6.6 RE) satellites. At the substorm onset, immediate and substantial energization of both protons and electrons in the plasma sheet was observed by the Imp 6 satellite in the midnight sector. This change was accompanied by a strong earthward plasma flow at ∼500 km/s for ∼12 min. However, no significant mid-latitude positive bays were detected, and no substorm injection of plasma sheet particles was observed by the ATS 5 satellite, located then at ∼0130 MLT. This observation indicates that substorms occurring on a contracted oval are accompanied by two of the characteristic substorm changes in the plasma sheet, namely, the energization of the plasma sheet particles and the occurrence of strong plasma flows, but the substorm injection does not necessarily reach the synchronous distance in the midnight sector. Further, it suggests that contracted oval substorms and intense substorms represent the same physical phenomenon, differing only in magnitude. It is also shown that after a prolonged quiet period the average energies of protons and electrons in the near-earth plasma sheet (at ∼11 RE) are as low as 2 keV and 100 eV, respectively.