J. Moen

Simultaneous observations of the cusp in optical, DMSP and HF radar data

A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.

Dayside auroral configurations: Responses to southward and northward rotations of the interplanetary magnetic field

We report specific types of daytime auroral activities occurring in response to certain rotations of the interplanetary magnetic field (IMF). The IMF rotations we focus on consist of a sharp southward turning from an initial northward orientation, followed by slower rotations back to an intermediate state, which is dominated by the By component (small Bz). When the IMF turned from north to south, the day side auroral configuration changed from one typical of northward conditions to one typical of southward conditions. The type 2 cusp aurora in the north (∼78°–79°magnetic latitude) was replaced by activation of a sequence of moving auroral forms (type 1 cusp aurora) at lower latitudes, each form appearing at the equatorward boundary of the previous one, resulting in ∼100- to 200-km total displacements of the auroral equatorward boundary in the cusp region during intervals of ∼5- to 10-min duration. This auroral activity is accompanied by enhancement of ionospheric convection and onset of magnetic bays on the ground. Since southward rotations of the IMF are expected to give rise to enhancements of the magnetopause reconnection rate, we conclude that these auroral/convection events represent distinct ionospheric signatures of such reconnection rate enhancements. When the IMF turned partially northward again, into an intermediate state, the aurora responded as expected, i.e., the equatorward auroral displacement was halted or reversed, and the type 2 aurora reappeared. The coexistence of the types 1 and 2 cusp auroras in the intermediate state is interpreted in terms of the simultaneous occurrence of reconnections at low and high magnetopause latitudes. The dynamical configurations of particle precipitation and ionospheric convection in the cusp region in such cases are discussed.

Dayside moving auroral forms and bursty proton auroral events in relation to particle boundaries observed by NOAA 12

This paper consider two sequences of moving auroral forms that were observed in the early postnoon sector above Svalbard. The series of events observed on January 12, 1992, moved westward (noonward) under unknown IMF conditions. The events observed on December 17, 1992, moved eastward while interplanetary magnetic field BY was strongly negative. The auroral luminosity for these events was dominated by the 630.0-nm emission line but correlated in space and time with subvisual Hs intensifications. NOAA 12 particle characteristics relate the moving auroral forms to low-latitude boundary layer precipitation located poleward of the electron trapping boundary, i.e., on open magnetic field lines. The auroral activity and associated particle signatures are both attributed to magnetopause reconnection. Since magnetic reconnection is the only known mechanism capable of bulk injecting magnetosheath ions and electrons on the dayside, we suggest that simultaneous occurrence of proton and electron auroral activity is a unique footprint of dayside magnetic reconnection.

Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic field

We report a study of ionospheric signatures of plasma entry and momentum transfer at the dayside magnetopause during northward oriented interplanetary magnetic field (IMF), combining ground observations of the dayside aurora and ionospheric ion drift (CUTLASS HF radar) with simultaneous particle precipitation data obtained from three overflights by the Defence Meteorological Satellite Program (DMSP) F12, F13 and F14 spacecraft. The observations were taken during a 37-min long interval of strongly northward IMF (Bz=7 nT; clock angle ∼10°–15°) after a rapid northward turning. The meridan scanning photometer at the ground station recorded a long stepwise poleward retraction and latitudinal widening of the band of auroral emission in the cusp region. Thus the activity includes a series of episodes which are characterized by an initial 1–2 min poleward “step” of the auroral poleward boundary, followed by a ∼3–4 min period of relatively steady auroral latitude. The auroral events were accompanied by bursts of “reverse” two-cell convection characterized by equatorward flow across the cusp poleward boundary. The three DMSP spacecraft, which traversed the poleward boundary of the cusp aurora from north to south, entered into a region of auroral precipitation where electrons and ions of magnetosheath origin were present, together with equatorward convection. The observations are found to be consistent with a theoretical description of a sequence of bursts of lobe reconnection involving both hemispheres. This process results in the capture of magnetosheath flux tubes and thereby closed flux is added to the dayside magnetosphere.

Capture of magnetosheath plasma by the magnetosphere during northward IMF

We interpret combined auroral and simultaneous particle data from an overflight of the ground station at Svalbard by the F13 DMSP spacecraft in terms of pulsed “capture” of northward-directed magnetosheath flux tubes by the magnetosphere, due to sequential lobe reconnection in both the southern and northern hemispheres. The event refers to a ∼40-min long interval characterized by strongly northward interplanetary magnetic field (IMF) (Bz=7 nT; clock angle ∼ 10°-15°). The meridan scanning photometer at the ground station records a long stepwise poleward retraction of the band of auroral emission in the cusp region. Each step is marked by an initial, brief poleward leap, followed by a ∼5 min period of relatively steady auroral latitude. In the one event where simultaneous particle data are available (at 1100 MLT), the spacecraft traverses a region of auroral precipitation where electrons and ions of magnetosheath origin are present, together with equatorward convection. With a large sunward IMF tilt (Bx=6-7 nT) in the winter hemisphere, we suppose that the process starts with reconnection poleward of the southern cusp followed by overdraped lobe flux which reconnects with magnetospheric field lines in the northern hemisphere.

Observations of simultaneous effects of merging in both hemispheres

In this event study, we have compared electric field measurements acquired near magnetic noon during a rocket flight from the SvalRak range with solar wind and interplanetary magnetic field (IMF) observations. The cusp is spatially bifurcated relative to its source regions. The data indicate that many effects observed at northern high latitudes were driven by dayside merging in the Southern Hemisphere, probably near the dawn side of the cusp. Applying the antiparallel merging criterion of Crooker [1979], we show that complex ground-based optical data are well ordered by considering that incoming interplanetary electric field phase fronts in the solar wind are tilted, allowing the two hemispheres to respond to the same elements of the solar wind stream at significantly different times. The data stream interacts first with the Southern Hemisphere at lag times significantly less than the simple advection time. Northern Hemisphere merging occurred later, the timing separation being related to the tilt and the strong IMF BX. Auroral emissions created by electrons injected from a Southern Hemisphere merging site may be located in close proximity to those from a Northern Hemisphere site, within the same all-sky image. With proper lag times established for the two source regions, it is clear that variations of dayside auroral emissions occur in response to small changes in the interplanetary electric field. The bifurcation is driven by IMF BY, while BX accentuates differences in the timing of the interaction. The detailed harmonization of distinct auroral features with interplanetary drivers strongly supports the utility of the antiparallel merging criterion in estimating when and where the solar wind and the magnetosphere interact. A similar ordering of auroral effects and in situ data with interplanetary variations cannot be achieved if merging proceeded at lower latitudes along a continuous, tilted merging line passing through the subsolar region, as required by the component-merging hypothesis. A consequence of merging limited to the high-latitude regions is that the smaller convection cell is driven by merging in the opposite hemisphere. While these conclusions are based on our analysis of a single interval and need independent confirmation, the investigation opens new possibilities for understanding cusp electrodynamics, implying a much greater solar wind/IMF control of magnetosphere-ionosphere phenomena than previously thought.

Dynamics of the aurora and associated convection currents during a cusp bifurcation event

We report the detailed two-dimensional evolution of a transient auroral bifurcation in the cusp region which occurred during an interval of eastward (By>0) oriented interplanetary magnetic field (IMF). The actual auroral bifurcation is characterized by the near-simultaneous activation and intensification of two latitudinally separated auroral forms which both expanded westward across the 1200 MLT meridian, after their initial appearance in the postnoon sector. The northernmost form (called type 2) is typical of periods of northward IMF orientation while the southernmost form (called type 1) represents a continuation and intensification of a type of auroral activity which is characteristic of intervals of southward IMF orientation. The auroral bifurcation is commonly observed when the IMF is in an intermediate regime of clock angles (∼45°–90°), i.e., when By is the dominating IMF component. We explain the transient cusp bifurcation in terms of a short-lived activation of high-latitude reconnection and an associated lobe cell convection/current system and type 2 aurora embedded in a long interval of active low-latitude reconnection/type 1 aurora.

Pulsating midmorning auroral arcs, filamentation of a mixing region in a flank boundary layer, and ULF waves observed during a Polar‐Svalbard conjunction

Magnetically conjugate observations by the HYDRA and the Magnetic Field Experiment instruments on Polar, meridian-scanning photometers and all-sky imagers at Ny-Alesund, and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers on November 30, 1997, illustrate aspects of magnetosphere-ionosphere coupling at 0900–1000 magnetic local times (MLT) and 70°–80° magnetic latitudes and their dependence on interplanetary parameters. Initially, Polar crossed a boundary layer on closed field lines where magnetospheric and magnetosheath plasmas are mixed. This region contains filaments where magnetospheric electron and ion fluxes are enhanced. These filaments are associated with field-aligned current structures embedded within the large-scale region 1 (R1) current. Ground auroral imagery document the presence at this time of discrete, east–west aligned arcs, which are in one-to-one correspondence with the filaments. Temporal variations present in these auroral arcs correlate with Pc 5 pulsations and are probably related to modulations in the interplanetary electric field. The auroral observations indicate that the filamented mixing region persisted for many tens of minutes, suggesting a spatial structuring. The data suggest further that the filamented, mixing region is an important source of the R1 current and the associated midmorning arcs. When the interplanetary magnetic field (IMF) turned strongly north, Polar had entered the dayside extension of the central plasma sheet/region 2 current system where it and the underlying ground magnetometers recorded a clear field line resonance of frequency ∼2.4 mHz (Pc 5 range). The source of these oscillations is most likely the Kelvin-Helmholtz instability. Subsequent to the IMF northward turning, the multiple arcs were replaced by a single auroral form to the north of Ny-Alesund (at 1000 MLT) in the vicinity of the westward edge of the cusp. ULF pulsation activity changed to the Pc 3–4 range in the regime of the pulsating diffuse aurora when the IMF went to an approximately Parker spiral orientation and the ground stations had rotated into the MLT sector of cusp emissions.

Unusual features of the January 1997 magnetic cloud and their effect on optical dayside auroral signatures

We study features of the January 1997 magnetic cloud and their effect on the optical aurora at 0630–0930 magnetic local time (MLT). WIND data suggest a thin plasma depletion layer (PDL) preceding the cloud, at whose outer edge the magnetic field rotates southwards, the proton temperature has a local maximum, and the dynamic pressure drops by a factor of 2. At the clouds front boundary there is a further dynamic pressure drop, another localized temperature rise, and a ∼30° field shear. Prior to arrival of the southward rotation at Earth, the ∼08 MLT aurora is dominated by forms with intense 557.7 nm emission, presumably of boundary plasma sheet origin, located south of zenith (∼75° MLAT) and moving eastward. Minutes after the southward rotation reaches Earth, this emission is replaced by an auroral form which encroaches into the field-of-view at ∼ 73°MLAT, expands southward to ∼70° MLAT and westward, approaching ∼0800 MLT at 0600 UT. Its latitudinal positions satisfy a known relation between cusp latitude and interplanetary BZ, and its morphological and spectral properties are similar to those previously associated with an energy-latitude dispersion signature attributed to plasma transfer at an open low latitude boundary layer. We argue that this auroral precipitation at such far off-noon MLTs may be due to enhanced reconnection occasioned by the strongly southward field in the PDL/cloud. A POLAR pass across the open/closed field line boundary at 66° invariant latitude is consistent with these inferences.

Excitation and decay of magnetospheric lobe cell convection and its associated aurora

We discuss multi-instrument observations of the excitation and decay within a ∼10 min interval of a convection pattern typical of lobe cells, centered at ∼1230 MLT in the winter hemisphere. The plasma convection and its associated aurora were triggered by a rapid northward turning of the interplanetary magnetic field (IMF) monitored by near-Earth spacecraft. The IMF stayed northward oriented ( Bx = −4 nT; By = 2–3 nT; Bz = 2–4 nT; clock angle = 30°–70°) for 8 min, before rotating back south. The optical instruments recorded the activation of an east-west aligned auroral form near the latitude of the pre-existing cusp aurora at ∼ 73° MLAT, which was followed by a smooth, 10-min long poleward advance at an average speed of 0.4 km s−1, reaching 75° MLAT at its most expanded phase. Simultaneous CUTLASS radar and local magnetic data confirm the occurrence of a large reconfiguration of plasma convection involving a change from antisunward to sunward flow components in the vicinity of the cusp, consistent with the presence of a lobe cell.