Ionospheric and geomagnetic responses to changes in IMF B_Z: a superposed epoch study

Superposed epoch studies have been carried out in order to determine the ionospheric response at mid-latitudes to southward turnings of the interplanetary magnetic field (IMF). This is compared with the geomagnetic response, as seen in the indices K p, AE and Dst. The solar wind, IMF and geomagnetic data used were hourly averages from the years 1967–1989 and thus cover a full 22-year cycle in the solar magnetic field. These data were divided into subsets, determined by the magnitudes of the southward turnings and the concomitant increase in solar wind pressure. The superposed epoch studies were carried out using the time of the southward turning as time zero. The response of the mid-latitude ionosphere is studied by looking at the F-layer critical frequencies, f o F2, from hourly soundings by the Slough ionosonde and their deviation from the monthly median values, δf o F2. For the southward turnings with a change in B z of δB z > 11.5 nT accompanied by a solar wind dynamic pressure P exceeding 5 nPa, the F region critical frequency, f o F2, shows a marked decrease, reaching a minimum value about 20 h after the southward turning. This recovers to pre-event values over the subsequent 24 h, on average. The Dst index shows the classic storm-time decrease to about −60 nT. Four days later, the index has still to fully recover and is at about −25 nT. Both the K p and AE indices show rises before the southward turnings, when the IMF is strongly northward but the solar wind dynamic pressure is enhanced. The average AE index does register a clear isolated pulse (averaging 650 nT for 2 h, compared with a background peak level of near 450 nT at these times) showing enhanced energy deposition at high latitudes in substorms but, like K p, remains somewhat enhanced for several days, even after the average IMF has returned to zero after 1 day. This AE background decays away over several days as the Dst index recovers, indicating that there is some contamination of the currents observed at the AE stations by the continuing enhanced equatorial ring current. For data averaged over all seasons, the critical frequencies are depressed at Slough by 1.3 MHz, which is close to the lower decile of the overall distribution of δf o Fl values. Taking 30-day periods around summer and winter solstice, the largest depression is 1.6 and 1.2 MHz, respectively. This seasonal dependence is confirmed by a similar study for a Southern Hemisphere station, Argentine Island, giving peak depressions of 1.8 MHz and 0.5 MHz for summer and winter. For the subset of turnings where δB z > 11.5 nT and P ≤ 5 nPa, the response of the geomagnetic indices is similar but smaller, while the change in δf o F2 has all but disappeared. This confirms that the energy deposited at high latitudes, which leads to the geomagnetic and ionospheric disturbances following a southward turning of the IMF, increases with the energy density (dynamic pressure) of the solar wind flow. The magnitude of all responses are shown to depend on δB z . At Slough, the peak depression always occurs when Slough rotates into the noon sector. The largest ionospheric response is for southward turnings seen between 15–21 UT.

EISCAT/CRRES observations: nightside ionospheric ion outflow and oxygen-rich substorm injections

We present combined observations made near midnight by the EISCAT radar, all-sky cameras and the combined released and radiation efects satellite (CRRES) shortly before and during a substorm. In particular, we study a discrete, equatorward-drifting auroral arc, seen several degrees poleward of the onset region. The arc passes through the field-aligned beam of the EISCAT radar and is seen to be associated with a considerable upflow of ionospheric plasma. During the substorm, the CRRES satellite observed two major injections, 17 min apart, the second of which was dominated by O+ ions. We show that the observed are was in a suitable location in both latitude and MLT to have fed O+ ions into the second injection and that the upward flux of ions associated with it was sufficient to explain the observed injection. We interpret these data as showing that arcs in the nightside plasma-sheet boundary layer could be the source of O+ ions energised by a dipolarisation of the mid- and near-Earth tail, as opposed to ions ejected from the dayside ionosphere in the cleft ion fountain.

The response of ionospheric convection in the polar cap to substorm activity

We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°-78°. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71° by the time of the expansion phase onset. A westward electrojet, centred at 65.4°, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1° at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.

Ionospheric Signatures of Pulsed Magnetopause Reconnection

The concept of zero-flow equilibria of the magnetosphere-ionosphere system leads to a large number of predictions concerning the ionospheric signatures of pulsed magnetopause reconnection. These include: poleward-moving F-region electron temperature enhancements and associated transient 630nm emission; associated poleward plasma flow which, compared to the pulsed variation of the reconnection rate, is highly smoothed by induction effects; oscillatory latitudinal motion of the open/closed field line boundary; phase lag of plasma flow enhancements after equatorward motions of the boundary; azimuthal plasma flow bursts, coincident in time and space with the 630nm-dominant auroral transients, only when the magnitude of the By component of the interplanetary magnetic field (IMF) is large; azimuthal-then-poleward motion of 630nm-dominant transients at a velocity which at all times equals the internal plasma flow velocity; 557.7nm-dominant transients on one edge of the 630nm-dominant transient (initially, and for large |By|, on the poleward or equatorward edge depending on the polarity of IMF By); tailward expansion of the flow response at several km s-1; and discrete steps in the cusp ion dispersion signature between the polewardmoving structures. This paper discusses these predictions and how all have recently been confirmed by combinations of observations by optical instruments on the Svalbard Islands, the EISCAT radars and the DMSP and DE satellites.

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.

Ionospheric signatures of pulsed reconnection at the Earth's magnetopause

THE plasma precipitating into the Earth's dayside auroral atmosphere has characteristics which show that it originates from the shocked solar-wind plasma of the magnetosheath1'2. The particles of the magnetosheath plasma precipitate down a funnel-shaped region (cusp) of open field lines resulting from reconnection of the geomagnetic field with the interplanetary magnetic field3. Although the cusp has long been considered a well defined spatial structure maintained by continuous reconnection, it has recently been suggested4–6 that reconnection instead may take place in a series of discontinuous events; this is the ‘pulsating cusp model’. Here we present coordinated radar and satellite observations of a series of discrete, poleward-moving plasma structures that are consistent with the pulsating-cusp model.

Dayside ionospheric convection changes in response to long-period interplanetary Magnetic field oscillations: Determination of the ionospheric phase velocity

Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north-south changes of the IMF. During the 6-hour interval in question IMF Bz oscillates between ±7 nT with about a 1-hour period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5° magnetic latitude, but separated by ∼2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (∼10 MLT) and then spread outward toward dawn and dusk with a phase speed of ∼5 km s−1 over the longitude range ∼8 to 12 MLT, slowing to ∼2 km s−1 outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds. We interpret these observations in terms of the ionospheric response to the onset, expansion and decay of magnetic reconnection at the dayside magnetopause.

Ionospheric convection and the substorm cycle

During substorms, magnetic energy is stored and released by the geomagnetic tail in cycles of growth and expansion phases, respectively. Hence substorms are inherently non-steady phenomena. On the other hand, all numerical models (and most conceptual ones) of ionospheric convection produced to date have considered only steady-state situations. In this paper, we investigate the relationship of substorms to convection. In particular, it is shown that the steady-state convection pattern represents an average over several substorm cycles and does not apply on time scales shorter than the substorm cycle period of 1-2 hours. The flows driven by the growth and expansion phases of substorms are integral (indeed dominant) part of, as opposed to a transient addition to, the overall convection pattern.

The excitation of ionospheric convection

This review presents recent observations of high-latitude ionospheric plasma convection, obtained using the EISCAT radar in the 'Polar' experiment mode. The paper is divided into two main parts. Firstly, the delay in the response of dayside high-latitude flows to changes in the interplanetary magnetic field is discussed. The results show the importance for the excitation of dayside convection of the transfer of magnetic flux from the dayside into the tail lobe. Consequently, ionospheric convection should be thought of as the sum of two intrinsically time-dependent flow patterns. The first of these patterns is directly driven by solar wind-magnetosphere coupling, dominates ionospheric flows on the dayside, is associated with an expanding polar cap area and is the F-region flow equivalent of the DP-2 E-region current system. The second of the two patterns is driven by the release of energy stored in the geomagnetic tail, dominates ionospheric flows on the nightside, is associated with a contracting polar cap and is equivalent to the DP-1, or substorm, current system. In the second half of the paper, various transient flow bursts observed in the vicinity of the dayside cusp are studied. These radar data, combined with simultaneous optical observations of transient dayside aurorae, strongly suggest that momentum is transferred across the magnetopause and into the ionosphere in a series of bursts, each associated with voltages of 30-80 kV. Similarities between these bursts and flux transfer events observed at the magnetopause are discussed.

DE-2 observations of filamentary currents at ionospheric altitudes

Conjunctive measurements made by the Dynamics Explorer 1 and 2 spacecraft on October 22, 1981, under conditions of southward IMF, suggest the existence of a cusp ion injection from a region at the magnetopause with a scale size of ∼ 1/2 to 1 R E . Current signatures observed by the LAPI and MAGB instruments on board DE-2 indicate the existence of a rotation in the magnetic field that is consistent with a filamentary current system. The observed current structure can be interpreted as the ionospheric signature of a flux transfer event (FTE). In addition to this large-scale current structure there exist three small-scale filamentary current pairs. These current pairs close locally and thus, if our interpretation of this event as an FTE is correct, represent the first reported observations of FTE interior structure at low-altitudes.

The ionospheric signatures of flux transfer events and solar wind dynamic pressure changes

The generation of flow and current vortices in the dayside auroral ionosphere has been predicted for two processes ocurring at the dayside magnetopause. The first of these mechanisms is time-dependent magnetic reconnection, in “flux transfer events” (FTEs); the second is the action of solar wind dynamic pressure changes. The ionospheric flow signature of an FTE should be a twin vortex, with the mean flow velocity in the central region of the pattern equal to the velocity of the pattern as a whole. On the other hand, a pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from, and almost orthogonal to, the motion of the whole pattern. In this paper, we make use of this distinction to discuss recent observations of vortical flow patterns in the dayside auroral ionosphere in terms of one or other of the proposed mechanisms. We conclude that some of the observations reported are consistent only with the predicted signature of FTEs. We then evaluate the dimensions of the open flux tubes required to explain some recent simultaneous radar and auroral observations and infer that they are typically 300 km in north–south extent but up to 2000 km in longitudinal extent (i.e., roughly 5 hours of MLT). Hence these observations suggest that recent theories of FTEs which invoke time-varying reconnection at an elongated neutral line may be correct. We also present some simultaneous observations of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (observed using the IMP8 satellite) and the ionospheric flow (observed using the EISCAT radar) which are also only consistent with the FTE model. We estimate that for continuously southward IMF ( ≈ 5 nT) these FTEs contribute about 30 kV to the mean total transpolar voltage (∼30%).

Transient reconnection: Search for ionospheric signatures

The concept of magnetic reconnection originated with the suggestion by Giovanelli [1946] that particles could be energized during solar flares near nulls in the magnetic field. Hoyle [1949] subsequently proposed that such a process could also act at nulls between the geomagnetic field and the interplanetary magnetic field (IMF) to generate the energized particles responsible for auroral displays. However, the idea of the interconnection of the two magnetic fields, as we know it today, was first presented by Hoyle's student, Dungey [1953, 1961].