LT Sector Research Articles

Comparison of model high‐latitude electron densities with Millstone Hill observations

Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high‐latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, including a dayside high density region, a nightside mid‐latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000–1900 LT sector) owing to the effect of horizontal transport. The nightside mid‐latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough was filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two‐cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall‐off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross‐polar‐cap potential. Efforts to reproduce the observed behavior by using a larger cross‐polar‐cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of the F region at high latitudes to the convection process.

Millstone Hill incoherent scatter observations of auroral convection over 60° < Λ < 75°: 3. Average patterns versus Kp

The Millstone Hill radar, Westford, Massachusetts (Λ = 56°), has been upgraded for observations in support of the International Magnetospheric Study (IMS) by means of the addition of a 150‐ft. diameter fully steerable antenna. This antenna allows measurements of F‐region ion drifts, from which F‐region electric fields can be derived, using the existing UHF (68‐cm wavelength) radar. This paper describes analytical models of the mean electric field behavior in the auroral zone over the interval 60° < Λ < 75° as determined from our measurements made during the year 1978. There was a greater preponderance of summer data in our 1978 observations and also more periods in which the inferred interplanetary magnetic field was directed toward the sun. A separate model is obtained for each of the three Kp ranges (< 2−, 2o to 3−, > 3o). Each model incorporates a polynomial description of intensity with invariant latitude and a sinusoidal harmonic variation in local time with 102 degrees of freedom for each of the perpendicular electric field components. Coefficients of this model are provided for numerical computation by users. Tabulations of these fields and a computer program to calculate fields and potentials are available on request from the authors. The results are compared with models published by Heppner [1977]. Each of these average field models is consistent with a two‐cell convection pattern having antisunward plasma flow over the pole and return paths eastward and westward through the dawn and dusk meridians. The centers of the vortices are near the far limit of observation (Λ = 75°). The entire convection pattern appears to be rotated by as much as two hours toward earlier local times from the nominal noon‐midnight over‐the‐pole flow direction, though this conclusion is influenced strongly by our least reliable (highest latitude) data. The dusk cell is dominant in size and strength; however, both the pattern rotation and cell dissimilarity lessen as Kp increases. The Harang discontinuity, which was recognizable on individual days, is largely absent in the average patterns, presumably as a result of the variability in its position. Nevertheless, we do see a residual, low‐speed, late‐evening protrusion of the dusk cell extending toward lower latitudes with increasing local time, reaching as far as 0300 LT for low Kp but only to midnight for high Kp. For low Kp, we observe a region of very small average flow from dawn to noon, apparently disconnected from the overall convection pattern. The dawn cell encroaches into this region as Kp increases, but the 1000–1200 LT sector maintains its low‐speed character even for high Kp. The cross‐polar‐cap potential varies linearly with Kp as V = 10.06 + 14.44 Kp kV, but the smoothing in our observations may have caused this to be an underestimate. Penetration of auroral electric fields to mid‐latitudes is most likely to occur and to be strongest in the evening sector.

EXOS‐B/Siple Station VLF Wave‐Particle Interaction Experiments: 2. Transmitter signals and associated emissions

Interactions between coherent VLF waves and energetic particles in the magnetosphere have been studied in a joint program involving the Japanese high‐altitude satellite EXOS‐B and the Siple Station VLF transmitter. During the period July 15–September 7, 1979, transmissions to the EXOS‐B satellite were carried out on 50 separate occasions when the spacecraft was within ±60° longitude of the magnetic field lines linking Siple Station, Antarctica (76°S, 84°W geographic, L ∼ 4.1), with its conjugate station at Roberval, Canada. Of this total, 37 were carried out while the satellite was located in the 1000–1600 LT sector of the magnetosphere and 13 while in the 0300–0800 LT sector. The transmitter signals were detected on EXOS‐B on 50% of the occasions when transmissions were attempted, and on 5 occasions the transmitter signals were observed to have triggered VLF emissions somewhere along their ray path between the ionosphere and the satellite. All 5 triggering events occurred in a 6‐day period following a large magnetic storm that took place on August 13, 1979, with 2 events occurring in the 1130–1330 LT sector and 3 events occurring in the 0400–0600 LT sector. Analysis of the emission triggering events provided strong evidence that the triggering took place inside whistler‐mode ducts and that the emissions reached the satellite only after being scattered at one end of the ducts by ionospheric irregularities. On at least one day, emissions were triggered by ducted echoes of the transmitter signals but not by the direct ducted pulses themselves. On another day a new type of triggered emission was observed. We conclude that in the noon sector of the magnetosphere the amplitude of nonducted signals from the Siple transmitter is generally less than the threshold level necessary for triggering in the nonducted mode.

Equatorially trapped plasma populations

Plasma observations made at 5.5RE. by the USCD detectors on the SCATHA satellite reveal the existence of a thermal plasma population trapped within a few degrees latitude of the magnetic equator, defined here as the minimum B surface. These measurements were restricted to the 1000 to 2000 LT sector by the spacecraft orbit. The ions show a higher degree of anisotropy than the electrons, with a FWHM of 10° to 25°, narrowing with increasing energy. The electron distribution shows a width of 20° to 60°, again narrowing with increasing energy. The 20‐ to 100‐eV ion fluxes typically show temperatures in the 20‐ to 50‐eV range, and densities of 1–10 cm−3. The electron population typically extends from 50 to 500 eV, with temperatures of 100‐200 eV and densities also in the 1‐ to 10‐cm−3 range. Lower energy field‐aligned populations are occasionally found in both ions and electrons at the same location. These could be sources for the warmer pancake plasma if heated primarily in the perpendicular direction. Equatorial noise in the 20‐ to 200‐Hz range is seen in the electric field data about 90% of the time in association with the trapped plasma, with an intensity corresponding to the strenth of the trapped fluxes.

Ion composition and energy distribution during 10 magnetic storms

Data from the plasma composition experiment on ISEE 1 were used to investigate the relative quantities and energy characteristics of H+, He++, He+, and O+ in the near‐equatorial magnetosphere at R ≤15 RE during magnetic storms, principally during the early main phase. The ions included in this study had energies in the range of 0.1 ≤E/Q ≤17 keV/e. The number densities were characterized by a large to dominant fraction of terrestrial ions throughout this energy window. Terrestrial O+ ions were most clearly identified, but strong evidence for a significant contribution of terrestrial H+ ions was also found. On occasions, the O+ alone contributed 50% or more of the integral number density, as well as the energy density, over distances of several earth radii along the orbit. The largest fractions of O+ (≲75%) and He+ (≲25%) were found at R <3 RE (L <5). In general, the He+ only represented a few percent, however. Small fractions of O+ (<10%) and He+ (<1%) were mostly found in the 0100‐0600 LT sector, at R ≳7 RE. The He++ was often obscured by background and rarely exceeded 2%, except in the 0100–0600 LT sector, at R ≳7 RE, where it reached several percent relatively frequently, suggesting a larger solar wind component here. It is argued, based on certain signatures in the energy spectra, that solar wind ions may enter the inner magnetosphere through this region and thereby contribute a larger portion of the high‐energy ring current population (50–100 keV). The data do not suggest, however, that the solar wind is always the dominant source of ions for the high‐energy ring current.

Proton precipitation during magnetic storms in the plasmapause region according to the data from the Intercosmos-5 satellite

The outer-zone >0.5 MeV proton behavior during magnetic disturbances is described according to measurements obtained from the low-altitude magnetically oriented satellite ‘Intercosmos-5’ for the period from December 1971 to April 1972. The satellite orbit allowed measurements to be made at invariant latitudes < 64°. During magnetic storms and Dst-depression events ( Dst≤30 γ) in the day 06–16 h LT sector the quasitrapped ( H min < 100 km) and enhanced trapped proton fluxes have been detected on L = 3–4.7. Both the precipitating (with pitch angle α≈0°) and quasitrapped protons have been found on the same L-shells in the evening and night (06–16 h LT) sectors. Analysis of the data has revealed that there are two zones of the storm-time proton appearance in the evening and night sectors. In the first zone, which is located at lower latitudes, most probably inside the plasmapause as projected to equatorial plane, only the quasitrapped protons appear. In the second zone, at higher latitudes, the proton fluxes were observed to be, as a rule, isotropic or with field-aligned anisotropy. The isotropic precipitation takes place, most probably, just outside the plasmapause. The coincidence between the position of the inner boundary of proton precipitation as measured by Intercosmos-5 and the plasmapause location as detected by Explorer-45 during the storm of 16 December 1971 is shown.

Modeling the midlatitude F-region ionospheric storm using east-west drift and a meridional wind

Under magnetically quiet conditions, ionospheric plasma in the midlatitude F-region corotates with the Earth and relative east-west drifts are small compared to the corotation velocity. During magnetic storms, however, the enhanced dawn-to-dusk magnetospheric convection electric field often penetrates into the midlatitude region, where it maps into the ionosphere as a poleward electric field in the 18:00 LT sector, producing a strong westward plasma drift. To evaluate the ionospheric response to this east-west drift, the time-dependent O + continuity equation is solved numerically, including the effects of production by photoionization, loss by charge exchange and transport by diffusion, neutral wind and E × B drift. In this investigation only the neutral wind's meridional component and east-west E × B drift are included. It is found that an enhanced equatorward wind coupled with westward drift produces an enhancement in the peak electron density (NMAX(F2)) and in the electron content (up to 1000 km) in the afternoon sector and a subsequent greater-than-normal decay in ionization after 18:00 LT. These results agree in general with midlatitude F-region ionospheric storm observations of NMAX(F2) and electron content which show an afternoon enhancement over quiet-time values followed by an abrupt transition to lower-than-normal values. Westward drift appears to be a sufficient mechanism in bringing about this sharp transition.

Whistler evidence of the dynamic behavior of the duskside bulge in the plasmasphere

Approximately 180 days of whistler data for 1963 and 1965 from Eights and Byrd, Antarctica, were reviewed for information on the bulge in the plasmasphere, the region of large plasmapause radius that is frequently found in the 18–21 LT sector. Previous findings on the essential permanance of the bulge and the abruptness of its westward end (involving an increase in plasmapause radius of 0.5–2.5 RE within 15–20 degrees longitude) were verified. The position in local time of the westward end of the bulge depends on the preceding history of substorm activity as evidenced in the auroral electrojet (AE) index. The bulge position was observed in the afternoon during particularly high levels of substorm activity, near 18 LT during more or less steady activity, and near 19–20 LT during periods preceded by 6–8 hours of quieting. The bulge is evidently released during periods of deep prolonged quieting and moves in the direction of the earth's rotation. The observations are interpreted in terms of sunward surges of the bulge plasma during substorms, followed by increased influence of the earth's corotation field during quieting. Several types of whistler evidence indicate shielding of the dayside plasmasphere from substorm convection fields. The plasmapause is not generally coincident with an equipotential of the combined magnetospheric flow, but it may approach an equipotential during relatively steady planetary conditions. Under such conditions the duskside radius of the plasmapause may provide a crude time-averaged measure of the stagnation distance and hence the intensity of the convection electric field in the dusk sector. Values of convection EYSM within the duskside plasmasphere, inferred from the stagnation distance and from motions of the bulge, are ∼1–4 mv/m during substorms, about 0.4–0.6 mv/m between substorms (but during moderate magnetic storms), and of order 0.1 mv/m during prolonged quiet periods. These values are larger by a factor of 3–4 than values of EYSM inferred from tracking cross-L drifts of whistler ducts in the plasmasphere near midnight.