G. B. Burns

Interhemispheric asymmetry of the high-latitude ionospheric convection pattern

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27-29, 1992. When the interplanetary magnetic field (IMF) Bz component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar-cap potential drops in the two hemispheres are similar. When Bz is positive and |By| > Bz, the convection configurations are mainly determined by By and they may appear as normal “two-cell” patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of |By|/Bz decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the “merging cell” potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The “lobe cell” provides a potential between 8.5 and 26 k V and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if |By| > |Bz|. To estimate the potential drop of the “viscous cells,” we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Diurnal Variations of Global Thunderstorms and Electrified Shower Clouds and Their Contribution to the Global Electrical Circuit

The long-standing mainstay of support for C. T. R. Wilson’s global circuit hypothesis is the similarity between the diurnal variation of thunderstorm days in universal time and the Carnegie curve of electrical potential gradient. This rough agreement has sustained the widespread view that thunderstorms are the ‘‘batteries’’ for the global electrical circuit. This study utilizes 10 years of Tropical Rainfall Measuring Mission (TRMM) observations to quantify the global occurrence of thunderstorms with much better accuracy and to validate the comparison by F. J. W. Whipple 80 years ago. The results support Wilson’s original ideas that both thunderstorms and electrified shower clouds contribute to the DC global circuit by virtue of negative charge carried downward by precipitation. First, the precipitation features (PFs) are defined by grouping the pixels with rain using 10 years of TRMM observations. Thunderstorms are identified from these PFs with lightning flashes observed by the Lightning Imaging Sensor. PFs without lightning flashes but with a 30-dBZ radar echotop temperature lower than 2108C over land and 2178C over ocean are selected as possibly electrified shower clouds. The universal diurnal variation of rainfall, the raining area from the thunderstorms, and possibly electrified shower clouds in different seasons are derived and compared with the diurnal variations of the electric field observed at Vostok, Antarctica. The result shows a substantially better match from the updated diurnal variations of the thunderstorm area to the Carnegie curve than Whipple showed. However, to fully understand and quantify the amount of negative charge carried downward by precipitation in electrified storms, more observations of precipitation current in different types of electrified shower clouds are required.

Characteristics of ionospheric convection and field-aligned current in the dayside cusp region

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

Variations of the atmospheric electric field in the near‐pole region related to the interplanetary magnetic field

Variations in the atmospheric, near-surface vertical electric field component Ez measured at the Russian Antarctic station Vostok in 1998 are analyzed in conjunction with changes of the interplanetary magnetic field (IMF). A total of 134 days were selected which satisfied the “fair weather” conditions, that is, days with absence of high winds, falling or drifting snow, clouds, and electric field “pollution” from the stations power plant. It is shown that the average diurnal variation of Ez for these days follows the global geoelectric field “fair - weather” diurnal variation: the “Carnegie” curve, which describes the global electric circuit formed by the thunderstorm activity occurring mostly over equatorial regions. The Ez diurnal variation shows strong seasonal dependence: it is maximal (∼40% of the average daily magnitude) in summer but gradually reduces through the equinoctial months and is almost negligible during the austral winter. Ez at Vostok is strongly affected by variations in both the IMF By and Bz components. The influence of By is dominant during geomagnetic daytime hours (1100–1400 UT at Vostok): Ez increases with By in the range from −10 to +10 nT. The IMF Bz effect is mainly seen at dawn (Ez decreases with Bz) and dusk (Ez increases with Bz).

Seasonal variations and inter-year trends in 7 years of hydroxyl airglow rotational temperatures at Davis station (69°S,78°E), Antarctica

Abstract Seasonal variation of hydroxyl rotational-temperatures above Davis, Antarctica (68.6°S, 78.0°E) is determined from 1148 nightly means collected over 7 years (1990 and 1995–2000). Measurements are limited by sunlight at this latitude to between day-of-year (DOY) 49 and 296. Sharp temperature transitions, particularly in autumn ( 1.2 K d −1 ), bracket an extended winter maximum with a slow temperature decrease ( −0.03 K d −1 from DOY 85 to 260). Seasonal variability and absolute magnitudes ( 206+/−4 K for a 1 July average across the years measured) are very similar to measurements at a comparable northern latitude (Lubken and von Zhan, 1991). A solar cycle association of 0.06 K ( sfu ) −1 ( 10.7 cm solar flux unit =10 −22 W m −2 Hz −1 ), implying a 7 K variation over a 120 sfu range solar cycle, fits the measured winter temperatures.

The influence of polar-cap convection on the geoelectric field at Vostok, Antarctica

Abstract Vertical geoelectric field measurements at Vostok, Antarctica ( 78.5° S , 107° E ; corrected geomagnetic latitude, 83.4°S) made during 1998 are compared with both Weimer (1996) and IZMEM (1994) model calculations of the solar-wind-induced, polar-cap potential differences with respect to the station. By investigating the correlations between these parameters for individual UT hours, we confirm and extend the diurnal range over which significant correlations have been obtained. Nineteen individual UT hours are significantly correlated with the Weimer model predictions and nine with the IZMEM model predictions. Diurnal variation in the slopes of the linear regressions allows us to comment on each model, demonstrating that Antarctic polar plateau geoelectric field measurements can be used to investigate polar convection. Seasonal variations in the diurnal electric field variations at Vostok are compared with the Carnegie global electric circuit diurnal curves, after allowance is made for the solar-wind-induced, polar-cap potential difference patterns.

The geoelectric field at Davis station, Antarctica

Abstract An electric field mill is used to measure the vertical component of the geoelectric field at Davis station, Antarctica (68.6°S, 78.0°S, geographic coordinates; 74.6°S magnetic latitude). Local influences on the measurements are determined. Approximately a year of data is subjectively examined to determine periods when the ‘fair-weather’ electric field is expected to be dominant. Using a ‘cumulation of consecutive differences’ method, small intervals of data are combined to determine winter, spring and autumn diurnal ‘fair-weather’ electric field curves. A paucity of intervals not locally influenced precludes determination of a summer diurnal curve. The seasonal-diurnal curves each show a peak between 19 UT and 22 UT that is similar in temporal location and relative magnitude to the global, fair-weather, seasonal diurnal curves (see Reiter, 1992, p. 130). A local influence persists between 03 UT and 10 UT and precludes determination of a magnetospheric influence on the geoelectric field for these data.

The geoelectric field at Vostok, Antarctica: its relation to the interplanetary magnetic field and the cross polar cap potential difference

The vertical geoelectric field measured at Vostok, Antarctica (78.5°S, 107°E, L=75.0) over the 13 month interval May 1979–May 1980 is correlated with the interplanetary magnetic field (IMF) components By and Bz at times when Vostok is connected to the dayside magnetosphere. No significant association with IMF Bx is found. The interaction of the solar wind and the Earth’s magnetic field generally results in anti-sunward plasma flow in the high-latitude, polar ionosphere driven by a dawn-to-dusk, cross polar cap potential difference pattern. Using the IZMEM model to infer the contribution of the cross polar cap potential difference to the potential difference between the ionosphere and the ground at Vostok for the measured IMF conditions, we show that this provides a viable mechanism for the IMF associations found. We demonstrate that the direct association of the geoelectric field with the cross polar cap potential difference is independent of a result (Park, 1976. Solar magnetic sector effects on the vertical atmospheric electric field at Vostok, Antartica. Geophysical Research Letters 3(8), 475–478) showing an ∼15% decrease in the vertical geoelectric field measured at Vostok, 1–3 days after the passage of IMF sector boundaries. Evidence is also presented supporting the Park result, for which a mechanism is yet to be confirmed.

Sources and velocities of Pc1‐2 ULF waves at high latitudes

The polar cusp and boundary layer are important in coupling magnetospheric energy sources to the high latitude ionosphere. ULF waves are one of the processes by which this coupling is realized. To study the source regions and propagation characteristics of discrete Pc1-2 (0.1–2 Hz) ULF wave packets, particularly unstructured emissions and Pc1 bursts at high latitudes, a triangular array of closely spaced induction magnetometers (∼150 km) was deployed beneath the average cusp projection during the 1992 Antarctic winter. From interstation time lags the wave velocity and direction of arrival were calculated with average uncertainties of ±60 kms−1 and ±8°. Wave sources were poleward of the array at low geomagnetic activity and equatorward at high activity. The sources also moved east to west with time, centred around local noon. These results are interpreted as indicative of the ionospheric signature of sources localized in the cusp, the low latitude boundary layer (LLBL) or the outer magnetosphere. Intercalibration of the results for typical events with extrapolations of PACE radar observations and DMSP satellite particle signatures support sources within these regions. Observed group velocities in the range 300–800 kms−1 with a mean of 450 kms−1 are consistent with wave propagation in the ionospheric waveguide. Signals above the waveguide lower cutoff frequency likely propagate away from the source in the ionospheric waveguide and across the magnetometer array. The results suggest a technique for monitoring the high latitude boundary regions and outer magnetosphere using local ULF wave measurements.