Plasma Convection Pattern Research Articles

A possible ground state configuration for plasmaspheric convection

Plasma convection patterns are compared for two 24 h periods of continuous whistler data recorded at Sanae (L = 4) under very quiet magnetic conditions. The plasma convection patterns are remarkably similar, both displaying a bulge at about 1700 UT (1530 MLT) and a second post-midnight bulge. It is therefore suggested that the double bulge in the convection pattern may be a permanent feature characteristic of ground state conditions.

Mean auroral E-region plasma convection patterns measured by SABRE

Large-scale flows of convective plasma in the Earth's magnetosphere, driven by the external influence of the solar wind, have long been known. The electric fields associated with this convection are mapped along approximately-equipotential field lines to low altitudes, resulting in strong ionospheric flows. Considerable controversy surrounds the detailed pattern of the convective flow in the ionosphere, its dependence on geomagnetic and interplanetary conditions and the resulting magnetospheric topology. Several observations1,2 suggest that this pattern rotates towards earlier local times during magnetically-disturbed periods, but empirically-based modelling failed to detect such an effect and even suggested the counter rotation, towards later local times3. Since April 1982, a new radar auroral backscatter system, SABRE (Sweden and Britain Radar auroral Experiment) has been in operation in north ern Europe4, which allows estimates to be made of plasma convection in the auroral E region over a large area (∼200,000 km2, L = 4–6) with high spatial and temporal resolution. Here we present the flow patterns averaged over this period of radar operation for different levels of magnetic activity, revealing a definite rotation of the whole convection pattern towards earlier local times with increasing activity.

F‐region ionospheric structure associated with antisunward flow near the dayside polar cusp

After one year of measurements using the Sondrestrom incoherent‐scatter radar (74° Λ), a number of repeatable features have been identified that interrelate the F‐region ionization and plasma convection patterns. One of these is an extended (poleward) ridge of ionization frequently present overhead near magnetic local noon. This ionization enhancement is associated with large ion drift velocities with a significant poleward component. Similar ionization features have been reported and thought to be produced by convection of F layer plasma through an ionization source—soft particle precipitation associated with the polar cusp. In this study, however, we are able to measure the electron temperature as well as number density and drift velocity. These measurements indicate that the electron temperature is not elevated within the density enhancement suggesting that the enhancement is not caused by local soft particle precipitation. Rather, it may simply be F‐region plasma convected from lower latitudes with correspondingly lower solar zenith angles.

Initial EISCAT observations of plasma convection at invariant latitudes 70°–77°

The potential of the EISCAT radar system for observing plasma convection patterns at high latitudes has been explored in a preliminary experiment on 27 November 1982. Using a beamswinging technique, plasma velocity was measured at slant ranges of 645–1170 km, enabling velocity vectors to be derived for invariant latitudes 70°–77°. Although operational problems limited the experiment to 80 min, some interesting observations of high-latitude flows were made. Typical afternoon westward flows of about 1 km s −1 were recorded over much of the interval, but important temporal and spatial variations were also seen as a result of the good time and space resolution of the experiment (5 min and 75 km, respectively). In particular, the westward flow was interrupted for about 10 min by a surge of poleward flow, possibly related to dynamic coupling occurring at the dayside magnetopause, such as a flux transfer event.

A theoretical F region study of ion compositional and temperature variations in response to magnetospheric storm inputs

The response of the polar ionosphere to magnetospheric storm inputs was modeled. During the “storm,” the spatial extent of the auroral oval, the intensity of the precipitating auroral electron energy flux, and the plasma convection pattern were varied with time. The convection pattern changed from a symmetric two‐cell pattern with a 20‐kV cross‐tail potential to an asymmetric two‐cell pattern with enhanced plasma flow in the dusk sector and a total cross‐tail potential of 90 kV. During the storm there were significant changes in the ion temperature, ion composition, and molecular/atomic ion transition height. The storm time asymmetric convection pattern produced an ion temperature hot spot at the location of the dusk convection cell owing to increased ion‐neutral frictional heating. In this hot spot there were significantly enhanced NO+ densities and hence molecular/atomic ion transition heights. During the storm recovery phase, the decay of the enhanced NO+ densities closely followed the decrease in the plasma convection speed. During the storm, elevated ion temperatures also appeared at high altitudes in the midnight‐dawn auroral oval region. These elevated ion temperatures were a consequence of the storm‐enhanced topside O+ densities, which provided better thermal coupling to the hot electrons. This region also contained reduced molecular/atomic ion transition heights. These elevated ion temperatures and reduced transition heights persisted for several hours after the storm main phase ended.

An analytic solution for the response of the neutral atmosphere to the high‐latitude convection pattern

We develop the analytic solutions to a two‐dimensional, shallow‐water model of the high‐latitude thermosphere. The equations are a linearized representation of the response of the neutral atmosphere to the momentum source due to the plasma convection pattern. Our results show how the neutral winds depend on the Pedersen and Hall ion drag coefficients and the horizontal wave number. The response depends on the dimensionless scale as it appears in the product of the Rossby radius of deformation and the wave number. The flow pattern is shifted in local time from the plasma flow pattern by an amount that depends on the average electron density. At the largest scales there is a 180° phase shift between the rotational flow components in the E and F regions. The model applies to quiet magnetic conditions and below 200 km of altitude.

Ionospheric hot spot at high latitudes

A hot spot (or spots) can occur in the high‐latitude ionsophere depending on the plasma convection pattern. The hot spot corresponds to a small magnetic local time‐magnetic latitude region of elevated ion temperatures located near the dusk and/or dawn meridians. For asymmetric convection electric field patterns, with enhanced flow in either the dusk or dawn sector of the polar cap, a single hot spot should occur in association with the strong convection cell. However, on geomagnetically disturbed days, two strong convection cells can occur, and hence, two hot spots should exist. The hot spot should be detectable when the electric field in the strong convection cell exceeds about 40 mV m−1. For electric fields of the order of 100 mV m−1 in the convection cell, the ion temperature in the hot spot is greatest at low altitudes, reaching 4000 °K at 160 km, and decreases with altitude in the F‐region. An ionospheric hot spot (or spots) can be expected at all seasons and for a wide range of solar cycle conditions.

A comparison of model predictions for plasma convection in the northern and southern polar regions

We have presented model calculations to show how the plasma flow distributions in the northern and southern polar regions differ when viewed from a geographic inertial frame. This reference frame was selected because it is the natural frame for geophysical plasma flow measurements, there being well‐known velocity corrections for either satellite or ground‐based observations. Although the magnetic invariant latitude, magnetic local time reference frame is better suited to studying magnetospheric processes, a transformation from the geographic inertial frame to this magnetic frame requires both a spatial and velocity transformation, and since the latter correction has generally been neglected, we prefer to present our results in the geographic inertial frame. However, we also present some of our results in the magnetic frame, taking account of the complete transformation. Our convection model includes the offset between the geographic and geomagnetic poles, the tendency of plasma to corotate about the geographic poles, and a dawn/dusk magnetospheric electric field mapped to a circle about a center offset by 5° in the antisunward direction from the magnetic pole. We considered both uniform and asymmetric magnetospheric electric field configurations. Our asymmetric electric field distribution contained an enhanced field in the dawnside northern hemisphere in conjunction with an enhanced field in the duskside southern hemisphere. From our study we have found the following: (1) In the geographic inertial frame the plasma flow patterns in both hemispheres exhibit significant variations with universal time because of the relative motion of the geomagnetic and geographic poles. (2) This universal time variation is greater in the southern polar region than in the northern polar region because of the greater displacement between the geomagnetic and geographic poles. (3) For the case of a uniform magnetospheric electric field the universal time dependence of the plasma flow distributions in the two hemispheres is similar, but there is a phase shift of about half a day between them. (4) For the case of an asymmetric magnetospheric electric field this half‐day phase shift is still noticeable, but there are significant differences between northern and southern hemisphere convection patterns. (5) The transformation of plasma convection patterns from the geographic inertial to the geomagnetic quasi‐inertial frame results in the same convection pattern for both hemispheres for the case of a uniform magnetospheric electric field, but results in different convection patterns for the two hemispheres for the more common case of an asymmetric electric field configuration. (6) Because the magnetospheric electric field distributions in the northern and southern polar regions are generally asymmetric, erroneous conclusions can be drawn about plasma convection patterns if data taken along satellite tracks from the northern and southern polar regions are overlaid. This is true whether the overlaying is done in the geomagnetic quasi‐inertial frame or the geographic inertial frame.

High latitude plasma convection: Predictions for Eiscat and Sondre Stromfjord

We have used a plasma convection model to predict diurnal patterns of horizontal drift velocities in the vicinity of the EISCAT incoherent scatter facility at TromsoNorway and for Sondre StromfjordGreenlanda proposed new incoherent scatter facility site. The convection model includes the offset of 11.4° between the geographic and geomagnetic poles (northern hemisphere)the tendency of plasma to corotate about the geographic poleand a magnetospheric electric field mapped to a circle about a center offset by 5° in the antisunward direction from the magnetic pole. Four different magnetospheric electric field configurations were consideredincluding a constant cross‐ tail electric fieldasymmetric electric fields with enhancements on the dawn and dusk sides of the polar capand an electric field pattern that is not aligned parallel to the noon‐midnight magnetic meridian. The different electric field configurations produce different signatures in the plasma convection pattern which are clearly identified. Both of these high‐latitude sites are better suited to study magnetospheric convection effects than either ChatanikaAlaska or Millstone HillMassachusetts. Alsoeach site appears to have unique capabilities with regard to studying certain aspects of the magnetospheric electric field.

Plasmasphere convection patterns observed simultaneously from two ground stations

Whistler data recorded during a 14 h period on 10–11 July 1973 at Siple ( L = 4.17) and Sanae ( L = 3.98) have been used to compare the apparent plasma convection patterns observed from these Antarctic stations. Two distinct bulges in the plasmasphere are seen at both stations, each bulge corresponding to an apparent outflowed followed by in flow of plasma. These structures do not coincide in U.T. or M.L.T. The first bulge is seen at Siple almost 1 h earlier in M.L.T. than at Sanae and the second bulge almost 3 h earlier. This is interpreted in terms of a fairly rapid westward and inward movement of the plasmasphere structure.

Plasmasphere response to the onset of quiet magnetic conditions: Plasma convection patterns

A whistler study has been made of plasma convection within the plasmasphere during a transition from steady moderate geomagnetic activity to quiet conditions. Continuous whistler data recorded at Sanae, Antarctica ( L= 3.98) for the period 0400 UT, 10 July to 0400 UT, 11 July 1973 have been analyzed in 15 min intervals. This study has revealed two distinct bulges in the plasmasphere centred on 1700 and 0100 UT. The bulges appear to result from the outward flow of plasma rather than the addition of new plasma. We tentatively interpret the late bulge at 0100 UT as being the duskside bulge of earlier studies rotated into the midnight region. In this bulge, plasma above L = 3.8 appears to convect outwards to form the bulge whereas plasma at lower L-values is relatively undisturbed. For the early bulge (1700 UT) the plasma convection pattern is similar over all observable L-values and closely reflects the shape of the estimated plasmapause in that region. Comparison of the bulges, with those obtained by Carpenter (1966) suggests that the onset of quiet conditions results in a general displacement of the bulges in an eastward direction by about 3 hr.

Electric field in the ionosphere and plasmasphere on quiet days

Observations of quiet day ionospheric electric fields from various middle- and low-latitude sources are analyzed by least squares fitting an electrostatic potential function to the data. The fit shows a reasonable amount of mutual consistency between the various data sources, but discrepancies point to the need for further study to delineate the universal time, seasonal, and day-to-day variations. Mapped into the magnetospheric equatorial plane, the electrostatic potential in the inner magnetosphere describes a plasma convection pattern on quiet days substantially different from that associated with a dawn-to-dusk electric field produced by solar wind-magnetosphere interactions.

Concluding remarks, session 5: Models, electric fields; session 6: Electric currents, late contributions

These two sessions, which cover the topics of E‐region models, electric fields, and electric currents, underscore a major difficulty necessarily encountered in studies of the E region. This difficulty arises from the fact that the E region of the ionosphere is just one part of a very large system, all of whose component parts are strongly coupled electrodynamically and hydrodynamically. Before describing a general approach to overcome this difficulty, let me briefly discuss some of the important coupling processes in the overall system.As the solar wind flows past the earth, it exerts a tangential stress at the magnetopause boundary. Information of this stress is propagated by hydromagnetic waves to the polar cap ionosphere, where an electric potential distribution is established that is consistent with the plasma motions required by the applied tangential stress. The resultant electric field pattern covers not only the polar cap region, but extends to the auroral zone and, to some extent, to middle and low latitudes. The electric fields imposed on the ionosphere equatorward of the polar cap are mapped into the closed field line regions of the magnetosphere, thus establishing a plasma convection pattern in the magnetosphere. The actual electric field pattern established is strongly affected by the magnetospheric plasma population, ionospheric conductivity, and neutral thermospheric winds. In fact, due to magnetospheric plasma effects, the electric field is largely shielded (on time scales of several hours or more) from the middle and low latitude regions. Nevertheless, relatively steady solar‐wind‐generated electric fields of a few mv m−1, along with much larger transient fields, probably are imposed at middle and low latitudes.

The magnetospheric radiation belt and tail plasma sheet

Theories of the origin of the storm-time radiation belt (the ring current) and associated bays and auroras have invoked hydromagnetic flow from the magnetotail on the one hand and flow into the tail on the other. It is shown that both flow patterns occur, the inward flow being the source of energy and driving the major magnetospheric convection pattern. Inward flow is the source of lower energy ( E ⪝ 10 keV ) auroral particles, of the asymmetric or storm-time radiation zone and higher energy ( E ⪞ 20 keV ) trapped particles, and of the permanent radiation belt itself. When the asymmetric belt develops sufficient energy density (⪞ 10 −8 erg cm 3 ) an electric space-charge field develops which changes the plasma convection pattern and ionospheric current system. The following phenomena are explained : (a) The tail plasma sheet is formed by the outward Hall drift of plasma from the stormtime radiation zone. (b) On a smaller scale, ‘bubbles’ of plasma from the radiation zone are released into the tail, accounting for the observed ‘islands’ of tail electrons, ‘spikes’ of electrons seen just above the ionosphere, and energetic protons (⪞ 300 keV) which have been seen in the tail. (c) The storm or bay ionospheric current system (DPI) develops from the quiet-time system (DP2) as a result of the magnetospheric space-charge field and changed ionospheric conductivity caused by auroral precipitation. (d) Quiet auroral arcs and auroral substorms are due to electron precipitation from regions of low magnetic field strength, where the first invariant (of trapped particles) is lost. The intermittent ejection of plasma from the radiation belt suggests that the model be termed the ‘burping radiation belt’.