D. M. Ober

Formation of density troughs embedded in the outer plasmasphere by subauroral ion drift events

A dynamic global core plasma model (DGCPM) is used to investigate the effects of subauroral ion drift (SAID) events on the formation of trough density profiles in the outer plasmasphere during periods of high magnetic activity. The DGCPM includes the influences of convection on the changing flux tube volumes, as well as daytime refilling and nighttime draining of plasma, to calculate the plasma tube contents and equatorial plasma density distribution versus time throughout the magnetosphere. SAIDs are regions of latitudinally narrow westward flow of plasma equatorward of the auroral zone. We present DGCPM results for various presumed SAID locations and durations relative to enhanced substorm convection onset and decay, to parametrically elicit the formation of plasmaspheric density trough structures resulting from SAID effects. It is found that imposing a SAID event in the dusk-evening sector for 30 min leads to the formation of a narrow (less than 1 RE near the equatorial plane) embedded plasma density trough in the dusk bulge region. The modeled plasmasphere density profiles with troughs generally resemble plasmasphere density profiles observed from DE 1 measurements.

Response of ionospheric convection to changes in the interplanetary magnetic field: Lessons from a MHD simulation

Characteristics of magnetospheric and high-latitude ionospheric convection pattern responses to abrupt changes in the interplanetary magnetic field (IMF) orientation have been investigated using an MHD model with a step function reversal of IMF polarity (positive to negative By) in otherwise steady solar wind conditions. By examining model outputs at 1 min intervals, we have tracked the evolution of the IMF polarity reversal through the magnetosphere, with particular attention to changes in the ionosphere and at the magnetopause. For discussion, times are referenced relative to the time of first contact (t = 0) of the IMF reversal with the subsolar nose of the magnetopause at ∼ 10.5 R E . The linear change in large-scale ionospheric convection pattern begins at t = 8 min, reproducing the difference pattern results of Ridley et al. [1997, 1998]. Field-aligned current difference patterns, similarly derived, show an initial two-cell pattern earlier, at t = 4 min. The current difference two-cell pattern grows slowly at first, then faster as the potential pattern begins to change. The first magnetic response to the impact of the abrupt IMF transition at the magnetopause nose is to reverse the tilt of the last-closed field lines and of the first-open field lines. This change in tilt occurs within the boundary layer before merging of IMF with closed magnetospheric field lines starts. In the case of steady state IMF By, IMF field lines undergo merging or changing partners with other IMF field lines, as they approach the nose and tilt in response to currents. When the By reversal approaches the magnetopause nose, IMF field lines from behind the reversal overtake and merge with those in front of the reversal, thus puncturing the reversal front and uncoupling the layer of solar wind plasma in the reversal zone from the magnetosphere. The uncoupled layer propagates tailward entirely within the magnetosheath. Merging of closed magnetospheric field lines with the new polarity IMF begins at t = 3 min and starts to affect local currents near the cusp 1 min later. While merging starts early and controls the addition of open flux to the polar cap, large-scale convection pattern changes are tied to the currents, which are controlled in the boundary layers. The resulting convection pattern is an amalgamation of these diverse responses. These results support the conclusion of Maynard et al. [2001b], that the small convection cell is driven from the opposite hemisphere in By-dominated situations.

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.

A semiempirical equatorial mapping of AMIE convection electric potentials (MACEP) for the January 10, 1997, magnetic storm

Owing to satellite and instrumental limitations, in situ magnetospheric electric field measurements are only available at isolated locations during storm time conditions. A global view of the inner magnetospheric convection electric field can be obtained by mapping ionospheric potentials into the equatorial plane. A mapping procedure for assimilative mapping of ionospheric electrodynamics (AMIE) ionospheric potentials (MACEP) is used to obtain convection patterns for the January 10, 1997, magnetic storm. The results are compared with the widely used empirical Volland-Stern model and the mapping of Weimer ionospheric potentials. While the gross temporal evolution of the large-scale potential drop across the magnetosphere is similar in all three models, detailed intercomparison shows that the MACEP procedure is capable of resolving highly variable and relatively small scale features of the electric field that are not treated by the Volland-Stern model nor seen from the Weimer mapping. The MACEP results are in reasonable agreement with limited electric field measurements from the electric field instrument on the Polar spacecraft and LANL measurements of thermal ion velocities at geosynchronous orbit during prestorm and recovery phase conditions. However, the inner boundary condition employed in the current version of AMIE is unable to reproduce the magnitude of the penetrating electric fields observed in the inner magnetosphere during the main phase of a storm. The addition of a penetration electric field associated with an asymmetric ring current in the dusk sector improved MACEP results at the duskside low-L region.

Mapping prenoon auroral structures to the magnetosphere

All-sky auroral images acquired at Ny-Alesund were used in conjunction with observations of a Polar overflight on November 30, 1997, to determine where prenoon, 0900-1000 magnetic local time (MLT) auroral structures map to the outer magnetosphere. Polar observations at midaltitudes are used to constrain the mapping between the aurora and the magnetosphere. The Tsyganenko 96 magnetic field model (T96), driven by interplanetary conditions and D ST , is used for that mapping. When the T96 model is driven by conditions on this day, the open/closed field line boundary maps 2-3 degrees lower in latitude than observed for this day. By making an ad hoc adjustment to match the location of the model open/closed field line boundary to observations, we find that the agreement with other aspects of the observations is also improved. These include (1) Polar observations of the dayside extension of the boundary plasma sheet (BPS) (structured low-energy electrons, detected in a region of sunward convection and region 1 field-aligned currents) mapped to the ionosphere in a region of discrete aurora, (2) Polar observations of the central plasma sheet (high-energy electron precipitation) maps to diffuse green auroral emissions equatorward of the discrete aurora, and (3) Polar electric field observations are consistent with convective motions of the auroral forms and changing interplanetary magnetic field (IMF) conditions. When the all-sky images are mapped into the magnetosphere, we find that the discrete aurora identified with the BPS mapped to the outer dawnside edge of the magnetosphere. This mapping indicates that earlier Geotail reports of a sunward flowing mixing region in the equatorial magnetosphere Fujimoto et al., 1998] is really within the dayside extension of the BPS, compatible with both the low-altitude BPS observations of Newell and Meng [1992] and type 4 dayside aurora of Sandholt et al. [1998]. Since the Polar observations place the sunward flowing mixing region as a source of region 1 currents for this IMF By positive case [Farrugia et al., this issue], these results extend the source of the region 1 currents from the antisunward flowing low-latitude boundary layer into the sunward flowing BPS, commensurate with Yamauchi et al. [1998] and Sonnerup [1980].

The relationship between suprathermal heavy ion outflow and auroral electron energy deposition: Polar/Ultraviolet Imager and Fast Auroral Snapshot/Time‐of‐Flight Energy Angle Mass Spectrometer observations

Ionospheric ions are energized to suprathermal energies (10–1000 eV) in the auroral zone. This produces a much larger quantity of escaping O+ ions than would otherwise occur, given typical ionospheric energies. Until recently, only limited work had been done relating ion upflow characteristics to nearby, contemporaneous auroral forms. We present our results comparing the characteristics of the suprathermal outflowing O+ ions, as measured by the Time-of-Flight Energy Angle Mass Spectrometer instrument on the Fast Auroral Snapshot (FAST) spacecraft, to the auroral forms seen at the foot point of the associated field line, as observed by the Ultraviolet Imager (UVI) on Polar. We present data from FAST nightside auroral zone passes between January 25 and February 11, 1997. During this interval, FAST made ∼100 auroral zone passes in the Northern Hemisphere where the aurora was simultaneously imaged by the UVI. Close examination of 50 such passes shows that the regions where suprathermal O+ outflow occurs closely follow the local aurora regardless of how convoluted the auroral forms may be. Taken together, these data show that the flux of escaping O+ ions increases by over a factor of 100 as the auroral intensity in the 1600–1800 A band increases from 0 to 4 kR. Also, the delay between auroral intensification and saturation O+ flux reaching 3000- to 4000-km altitude is ∼5–10 min.

Electrodynamics of the poleward auroral border observed by Polar during a substorm on April 22, 1998

Observations from Polar during a substorm on April 22, 1998, are used to specify electrodynamic characteristics of the high-latitude auroral boundary on the nightside. Polar was moving equatorward near invariant latitude 72°, 2305 magnetic local time as it crossed the auroral boundary near the end of the substorms expansion phase. This boundary was marked by severe east-west plasma flow shears, a reversal of the in-track electric field component, and multiple field-aligned currents. Harmonizing ground measurements with auroral images and in situ particle and field data from Polar reveals five electrodynamic features of the boundary. (1) A 20-min delay occurred between substorm onset and when the total magnetic flux in the polar cap began to decrease. This represents the time that elapsed before reconnection of open lobe flux began along a near-Earth X-line. (2) The reconnection electric field at the ionospheric projection of the X-line ranged between 20 and 70 mV m−1. Reconnection was intermittent, turning on and off at different locations. (3) Electric and magnetic field structures observed by Polar suggest that Alfven waves propagating along the auroral boundary carried a double-layer current. Downward Poynting flux was observed at the poleward auroral boundary associated with these currents. (4) Magnetic and electric field oscillations with periods of ∼90 s were detected on open field lines beginning ∼4 min before Polar entered the auroral oval. Oscillations with similar frequencies were observed both on the ground near Polars magnetic footprint and at geosynchronous orbit. This indicates that the oscillations represent a large-scale phenomenon occurring over a large portion of the nightside magnetosphere. Coupling on open field lines derives from fringing fields associated with ionospheric closure of DP 1 currents. (5) Upward flowing hydrogen and oxygen ions were detected at and equatorward of the auroral boundary. Perpendicularly accelerated O+ ions detected in the immediate vicinity of the boundary can be explained by direct acceleration by the ambient electric field perpendicular to the local magnetic field. Equatorward of the boundary, O+ distributions were typical of ion conics.

Premidnight plasmaspheric “plumes”

To explain observations of brief intervals of cold, dense plasma by geosynchronous satellites in the midnight sector prior to or during substorm onset, it has recently been proposed that dense plasmaspheric plasma is drawn out to geosynchronous orbit in the premidnight region by inductive electric fields during the growth phase of a geomagnetic substorm. We present here the results of a statistical study of such intervals observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellite 1989-046 between March and December 1993. We find that these premidnight cold plasma intervals occur only after extended periods of low magnetospheric activity identified by Kp and the midnight boundary index (MBI). We also find that the satellite typically enters the cold plasma region from the trough region and exits it into the plasma sheet. Finally, while measurements of the flow velocity of the cold plasma are rendered uncertain by the asymmetric spacecraft charge or photoelectron sheath, such measurements show no evidence of the outward flow that would be expected from the extrusion hypothesis. Rather, there are some indications that these cold plasma regions flow sunward in the (corotation) satellite frame. These results suggest an alternative explanation for the premidnight cold plasma: corotation-dominated transport of day side plasmaspheric structures into the premidnight sector. The implications of our observations for the extrusion hypothesis and for the alternative explanation are discussed.

Observations of bow shock and magnetopause crossings from geosynchronous orbit on 31 March 2001

[1] On 31 March 2001, significant enhancements in both the strength of the interplanetary magnetic field and the solar wind dynamic pressure pushed the magnetopause and bow shock inside of geosynchronous orbit. We present here observations of the unshocked solar wind observed with the Los Alamos magnetospheric plasma analyzer on geosynchronous satellite 1994-084 near local noon. The magnetosheath was observed intermittently at geosynchronous satellites 1991-080, 1994-084, and LANL-01A, across much of the dayside magnetosphere. Comparisons with theory of the observed bow shock and magnetopause locations at geosynchronous orbit near noon are in reasonable agreement. Using nearly simultaneous observations of the magnetopause and bow shock locations at 0451 UT, we have tested models for the thickness of the magnetosheath and flaring of the magnetopause at that time. Using the least flared of the magnetopause models, the estimated thickness of the magnetosheath would be thicker than would be predicted by either the gas dynamic solution of Spreiter et al. [1966] or the MHD simulations of Cairns and Lyon [1996]. Alternatively, placing the location of the magnetopause at the nose of the magnetosphere in conformance with the thickness of the magnetosheath from Cairns and Lyon [1996] requires less flaring on the flanks of the magnetosphere than currently present in the empirical models. The models also have difficulty predicting the other observed magnetopause crossings far from local noon apparently due to excessive flaring of the models for the conditions on this day.

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.