Fredrick J. Rich

Lobe cell convection as a summer phenomenon

Patterns of average potential over the high-latitude ionosphere in winter show that the dusk convection cell dominates the dawn cell, consistent with the presence of a day-night conductivity gradient, as predicted by a number of models. However, in the summer hemisphere, when IMF B{sub y} is strongly positive, the dusk cell so dominates the dawn cell that the latter nearly disappears; and when IMF B{sub y} is strongly negative, the cells are most nearly equal. The difference between winter and summer can be explained by the addition in summer of a single lobe cell, that is, a cell confined to open field lines, circulating within the dusk cell of the two-cell pattern when B{sub y} is positive and within the dawn cell when B{sub y} is negative. The result is consistent with predictions of the overdraped lobe model, that lobe cells occur in only one hemisphere at a time, and that their occurrence is controlled by dipole tilt. 35 refs., 3 figs.

Comparison of global MHD simulations with AMIE simulations for the events of May 19–20, 1996

Using WIND-measured solar wind data, we have simulated the magnetosphere for the time between 1200 UT May 19 and 0200 UT May 20, 1996, with a three-dimensional MHD model. This time period has been chosen as an International Solar-Terrestrial Physics/Global Geospace Science event for community study, and there is a large set of data with which to compare. In this paper we will compare the simulation predictions with results from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) analysis. We show comparisons for the convection, the auroral precipitation, the ionospheric conductances, the field-aligned currents, and the Joule heating distribution. The results concentrate on four time periods when the two DMSP spacecraft, F12 and F13, and the POLAR spacecraft were passing over the northern (summer) polar region. The comparisons show excellent agreement with the F13 electric field measurements. The ionospheric convection patterns agree well between the simulation and the AMIE analysis with the cross polar potential drop somewhat higher in the MHD model. The auroral electron precipitation energy flux from the MHD model is too low, particularly in the late morning, when compared with the POLAR UVI data because of the lack of electron drift physics in the model. We show how the MHD auroral input can be improved by adjusting the parameters in the auroral precipitation model.

Solar activity variations in the composition of the low‐latitude topside ionosphere

Ion composition data from the retarding potential analyzer onboard the Defense Meteorological Satellite Program (DMSP) F10 have been analyzed for the months of June and September for the years 1991–1994 when the solar F10.7 flux changed from near 200 to less than 100. Low-latitude composition data have been averaged by dip latitude and geographic longitude for morning and evening passes to investigate variations attributable to solar variability. In 1991 the dominant ion is O+ in both the morning and the evening, but by 1994, O+ is the dominant ion only at certain locations in the morning. The O+ −H+ transition height is well above the DMSP altitude (800 km) in 1991, but the transition height is near or below 800 km in 1994. Neutral wind induced longitude variations in the topside are present under all levels of solar activity, but the differing role of interhemispheric plasma transport at different solar activity levels dramatically changes the latitude distributions resulting from the winds. At high solar activity the O+ −H+ transition height is well above 800 km, and interhemispheric transport of O+ in the flux tubes connecting the northern and southern hemispheric topside reduces the latitude asymmetry in O+ while producing the minimum observable H+ asymmetries at night. At lower solar activity levels, H+ dominates the topside ionosphere above 800 km, and larger latitudinal asymmetries in the O+ concentration are observed, while the H+ latitude distribution remains quite symmetric at all local times.

DMSP F8 observations of the mid‐latitude and low‐latitude topside ionosphere near solar minimum

The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840-km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light-ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field-aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E {times} B drifts qualitatively explain most features of these composition and velocity measurements. 23 refs., 5 figs., 2 tabs.

A multievent study of broadband electrons observed by the DMSP satellites and their relation to red aurora observed at midlatitude stations

Broadband electrons during magnetic storms are characterized by an unusually intense flux of precipitating electrons in the broadband energy range from 30 eV to 30 keV near the equatorward edge of the auroral oval (47°–66° magnetic latitude). Broadband electrons were first reported by Shiokawa et al. [1996]. In this paper, we report a multievent study of broadband electrons, using particle data obtained by the Defense Meteorological Satellite Program (DMSP) satellites during 23 magnetic storms from January 1989 through May 1992. Twelve broadband electron events are identified. Most of them are observed in the night sector, but some are observed in the morning sector. Particle data for successive polar passes of the DMSP multisatellites are used to show that broadband electrons generally last for less than 30 min and that for some events, they precipitate over a wide range of local times simultaneously. On the basis of a quantitative calculation of optical emissions from electrons in the neutral atmosphere, we conclude that broadband electrons are a possible cause of red auroras observed at midlatitude ground stations. We suggest that broadband electrons are associated with certain substorms during the main phase of magnetic storms. This conjecture comes from observations of H component positive bays and Pi 2 pulsations observed at low-latitude magnetic stations and from magnetic field variations observed at geosynchronous satellites. We conclude that the magnetospheric source of broadband electrons lies within the inner part of the plasma sheet. This conclusion is based on the facts that broadband electrons appear in latitudes where plasma sheet particles were observed before the event and that broadband electrons are observed poleward of the subauroral ion drifts, a position that corresponds to the inner edge of the injected particle layer during storms. High-energy particle data obtained at geosynchronous satellites show that both strong magnetopause compressions in the dayside and decrease of the particle fluxes in the nightside occur in association with the broadband electron events. Possible mechanisms of broadband electron production are discussed.

Diurnal variation of the dayside, ionospheric, mid-latitude trough in the southern hemisphere at 800 km: Model and measurement comparison

Abstract Our high latitude ionospheric model predicts the existence of a pronounced “dayside” trough in plasma concentration equatorward of the auroral oval in both the Northern and Southern Hemispheres for solar maximum, winter, and low geomagnetic activity conditions. The trough in the Southern Hemisphere is much deeper than that in the Northern Hemisphere, with the minimum trough density at 800 km being 2 × 10 3 cm −3 in the Southern Hemisphere and 10 4 cm −3 in the Northern Hemisphere. The dayside trough has a strong longitudinal (diurnal) dependence and appears between 11:00 and 19:00 U.T. in the Southern Hemisphere and between 02:00 and 08:00 U.T. in the Northern Hemisphere. This dayside trough is a result of the auroral oval moving to larger solar zenith angles at those universal times when the magnetic pole is on the antisunward side of the geographic pole. As the auroral ionization source moves to higher geographic latitudes, it leaves a region of declining photoionization on the dayside. For low convection speeds, the ionosphere decays and a dayside trough forms. The trough is deeper in the Southern Hemisphere than in the Northern Hemisphere because of the greater offset between the geomagnetic and geographic poles. Satellite data taken in both the Northern and Southern Hemispheres confirm the gross features of the dayside trough, including its strong longitudinal dependence, its depth, and the asymmetry between the Northern and Southern Hemisphere troughs.

Quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector: A case study

This is the first paper which reports the characteristics of quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector. The moving arcs are observed from ground-based stations at magnetic latitudes (MLAT) of 78° and 84° during magnetically quiet intervals (interplanetary magnetic field Bz ∼ 0 or > 0). The arcs move poleward repeatedly with a period of several minutes and a velocity of ∼400–500 m/s and disappear at around 85° MLAT. For the event observed at 78° MLAT, the arcs are repeatedly detached from a stable aurora which is located at the equatorward of the arcs. The moving arcs correspond to accelerated electrons observed by the Exos D satellite. The stable aurora corresponds to continuous precipitation of high-energy electrons which probably originate from the inner part of the plasma sheet. The ion drift data from the DMSP-F11 satellite show that the poleward moving arcs are located around the boundary of the large-scale sunward flowing region at lower latitudes and the antisunward flowing region at higher latitudes. From these results, we conclude that the arcs are connected to the boundary region between the plasma sheet and the low-latitude boundary layer in the morningside tail flank. Several mechanisms which can produce the observed motions of the arcs are discussed.

Average Height-Integrated Joule Heating Rates and Magnetic Deflection Vectors Due to Field-Aligned Currents during Sunspot Minimum

Abstract Height-integrated Joule heating has been calculated from simultaneous observations of field-aligned currents and precipitating electrons made by the Defense Meterorological Satellite Program (DMSP) satellite F7. The DMSP/F7 satellite provided nearly continuous data from January 1984 to October 1987. In this paper we use data from January 1984 to December 1985, a period of nearly uniform, low solar EUV flux near the minimum of the sunspot cycle. By assuming that the majority of the observed field-aligned currents connect through the ionosphere via local Pedersen currents, we have calculated the local Joule heating rates. By combining Joule heating observations from multiple orbits, a survey of Joule heating vs magnetic latitude, magnetic local time, magnetic activity level and season has been compiled. In addition, the average magnetic deflection vector has been compiled. Our survey of the distribution of Joule heating has finer resolution than previous surveys, and is comparable with previous case studies. We have found evidence that the source of the field-aligned current into/out of the dayside cusp is on open field lines and that the source of the field-aligned current into/out of the auroral zones is on closed field lines.

High latitude electrodynamics: Observations from S3-2

The high spatial-temporal resolution of instrumentation on the polar-orbiting S3-2 satellite has allowed a wide variety of measurements of the electrodynamic characteristics of both large- and small-scale structures at high latitudes. Analyses of large scale features observed by S3-2 have shown that: (i) The IMF Bydependence of polar cap convection, first observed in June 1969 by OGO-6 persists in other seasons. During periods of northward IMF Bzextensive regions of sunward convection may be found in the sunlit polar cap. (ii) In the dawn and dusk MLT sectors >90% of the region 1 currents lie equatorward of the convection reversal line. Potentials across the ionospheric projection of the low-latitude boundary layer are typically a few kV. (iii) The location of ‘extra’ field-aligned currents, near the dayside cusp and poleward of the region 1 current sheet is dependent on the IMF Bycomponent. (iv) Simultaneous observations by TRIAD and S3-2 show that sheets of field-aligned current extend uniformly for several hours in MLT, but may have an altitude dependence in the 1000–8000 km range. (v) During magnetic storms ionospheric irregularities occur in regions of poleward density gradients and downward field-aligned currents near the equatorward boundary of diffuse auroral precipitation. In the winter polar cap, density irregularities were also found in regions of highly structured electric fields and soft electron precipitation. (vi) During an intense magnetic storm the auroral zone height-integrated Pederson conductivity was calculated to be in the range 10–30 mho and downcoming energetic electron fluxes accounted for between 50% and 70% of the upward Birkeland currents.Analysis of small-scale structures (latitudinal width < 1°), observed by S3-2, have shown that: (i) Intense meridional electric fields (50–250 mV m-1) generated by charge separation near the inner edge of the plasma sheet drive intense subauroral convection and are associated with field-aligned currents, on the order of 1–2 μA m-2. (ii) Case studies of discrete arcs in the auroral oval have shown that arcs are associated with pairs of small-scale, field-aligned currents embedded in the large-scale region 1/region 2 field-aligned current sheets. The maximum observed field-aligned current was an upward current of 135 μA m-2, confined to a latitudinal width of ∼ 2km and carried by field-aligned accelerated electrons. Return (downward) currents associated with arcs are limited to intensities of 10–15 μA m-2. At this limit the ionospheric plasma becomes marginally stable to the onset of ion-cyclotron turbulence. Two instances of plasma vortices, characteristic of auroral curls, have been observed in the region between the paired current sheets. (iii) Sun-aligned arcs in the polar cap are found in a region of negative electric field divergence, embedded in an irregular electric field pattern. The electrons producing the arcs have a temperature of ∼ 200 eV and have been accelerated through potential drops of ∼ 1 kV along the magnetic field. Return currents may appear on both sides of polar-cap arcs.

Simultaneous DMSP, All-Sky Camera, and IMAGE FUV Observations of the Brightening Arc at a Substorm Pseudo-Breakup

Auroral particles, field-aligned currents, and plasma convections in the vicinity of the brightening arc at substorm onset are still not well understood, since it is very rare to have conjugate satellite measurements above the brightening arc. In this paper, we investigate the characteristics of auroral particles and fields associated with the brightening arc at a pseudo-onset of substorm on October 31, 2000, using ground all-sky TV images, IMAGE FUV auroral images, and particle, magnetic field, and plasma flow data obtained by the DMSP F12 satellite. The arc brightening at Tixie (66.0°MLAT), Russia, occurred at 1004 UT (18.75 MLT) coincident with a coherent Pi 2 pulsation at midlatitudes and with the DMSP crossing above the arc. The brightening arc did not develop on a global scale, indicating that this event is a pseudo auroral breakup, which occurred ≈16 min before the major substorm expansion onset. IMAGE auroral images indicate that the longitude of the brightening center was ≈2.5 h nightside of Tixie. The DMSP data show that the precipitating particles associated with the brightening arc correspond to an electron inverted-V structure at the equatorward edge of the electron precipitation region. The arc was located in the energetic (>1 keV) ion precipitation region, near the equatorward boundary of the upward region 1 field-aligned current, and at the peak of the sunward convection velocity. These facts indicate that the brightening arc at duskside of the onset local time was located in the inner plasma sheet at the inner edge of the region 1 current source in the sunward convection region.