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- Simultaneous Observations of E-Region Coherent Backscatter and Electric Field Amplitude at F-Region Heights with the Millstone Hill UHF Radar
- Mid-Latitude Ionospheric Perturbation Associated with the Spacelab-2 Ionospheric Depletion Experiment at Millstone Hill
- Prompt Mid-Latitude Electric Field Effects during Severe Geomagnetic Storms
- Mid-Latitude Particle and Electric Field Effects at the Onset of the November 1993 Geomagnetic Storm
- Storm Time Plasma Transport at Middle and High Latitudes
- Coordinated SAR Arc Observations: Relationship to Plasma Convection
- Radar Observations of Magnetosphere-Ionosphere Coupling at Mid and High Latitudes
- Russian-American Tomography Experiment
- Aspect Angle Variations in Intensity, Phase Velocity, and Altitude for High-Latitude 34-cm E Region Irregularities
- Storm Time Electric Field Penetration Observed at Mid-Latitude
- High-Resolution Backscatter Power Observations of 440-MHz E Region Coherent Echoes at Millstone Hill
- Phase Velocity Studies of 34-cm E Region Irregularities Observed at Millstone Hill
Return to John Foster's Homepage
A complete list of Atmospheric Sciences Group publications is included in the Millstone Hill Observatory Bibliography.
J. C. Foster, P. J. Erickson
A combined coherent backscatter - incoherent scatter experiment was performed with the Millstone Hill UHF radar during strongly disturbed conditions on August 27, 1998 which provided simultaneous observations of electric field magnitude and coherent backscatter parameters on the same L shell. A carefully-designed geometry used sidelobe coherent contamination from two-stream irregularities at 110-km altitude, appearing at ranges corresponding to F-region altitudes in the main beam, in conjunction with simultaneous uncontaminated F-region observations of the E ¥ B drift velocity in adjacent range gates. Three hours of such observations with 20-s temporal resolution were analyzed during varying conditions in which |E| varied [30, 80 mV/m]. We find a clear linear relationship between both logarithmic coherent power and the magnitude of the coherent phase velocity with |E|. The results reported here are the first such observation of the relationship of coherent parameters to electric field magnitude made at 440 MHz and with the radar k ~ parallel to Vph. With the assumption that the coherent phase velocity is approximately the perturbed ion sound speed in the heated E region, we find an excellent agreement between the electron temperature derived from Vph and previous incoherent-scatter results relating E-region Te to |E|. This confirms that heating by two-stream waves is the mechanism responsible for electric field-dependent electron temperature enhancements in the absence of energetic electron precipitation. A maximum value of ~3100° K has been found for such wave-induced E-region heating. return to List of Abstracts
J. C. Foster, J. M. Holt, L. J. Lanzerotti, M. Mendillo
Elevation scans across geomagnetic mid latitudes by the incoherent scatter radar at Millstone Hill captured the ionospheric response to the firing of the Space Shuttle Challenger OMS thrusters near the peak of the F layer on July 30, 1985. Details of the excitation of airglow and the formation of an ionospheric hole during this event have been reported in an earlier paper [Mendillo et al., 1987]. The depletion (factor ~2) near the 320 km Shuttle orbital altitude persisted for ~35 min and then recovered to near normal levels, while at 265 km the density was reduced by a factor of ~6; this significant reduction in the bottomside F-region density persisted for more than 3 hours. Total electron content in the vicinity of the hole was reduced by more than a factor of 2, and an oscillation of the F-region densities with 40-min period ensued and persisted for several hours. Plasma vertical Doppler velocity varied quasi-periodically with a ~80-min period, while magnetic field variations observed on the field line through the Shuttle-burn position exhibited a similar ~80-min periodicity. An interval of magnetic field variations at hydromagnetic frequencies (~95 s period) accompanied the ionospheric perturbations on this field line. Radar observations revealed a downward phase progression of the 40-min period density enhancements of -1.12° km-1, corresponding to a 320-km vertical wavelength. An auroral-latitude geomagnetic disturbance began near the time of the Spacelab-2 experiment in response to the imposition of a strong southward IMF Bz across the magnetosphere, creating an additional complication in the interpretation of the active ionospheric experiment. It cannot be determined uniquely whether the ionospheric oscillations which followed the Spacelab-2 experiment were related to the active experiment or were the result of a propagating ionospheric disturbance (TID) launched by the enhanced auroral activity. The pronounced depletion of the bottomside ionosphere, however, accentuated the oscillatory behavior during the interval following the Shuttle OMS burn. The hydromagnetic waves likely resulted from the onset of the magnetospheric substorm. return to List of Abstracts
J. C. Foster
Meridian-plane elevation scans with the Millstone Hill incoherent scatter radar provide evidence of a strong perturbation of the coupled mid-latitude magnetosphere-ionosphere system during the early phases of the November 4, 1993 magnetic storm. A narrow ionospheric trough formed at L=3.5 in the pre-midnight sector, immediately poleward of the Millstone Hill site. The most pronounced radar signature of the developing activity was a brief (20 min) uplifting of the F region plasma equatorward of the trough, such that the peak altitude increased with distance away from the trough. A similar signature had been observed during storm onset on March 20, 1990, and in that event a pronounced topside ionospheric depletion developed in the region far equatorward of the mid-latitude trough and was observed by the radar and the DMSP F9 satellite. During the November 4, 1993 event, the DMSP F10 satellite observed narrow, magnetically conjugate regions of plasma density depletion and strong horizontal and upward plasma velocity (> 1500 m/s) at L=1.5 at the time of the uplifting of the mid-latitude F region observed by the radar. These observations were confined to longitudes near the South Atlantic magnetic anomaly and, in the Nov 1993 case, the perturbation was coincident with the peak of the precipitating particle fluxes associated with inner-belt losses at the anomaly. Both the uplifting of the ionospheric F layer and the triggering of topside density perturbations can be explained in terms of an eastward electric field imposed on the mid and low-latitude ionosphere during the initial stages of the geomagnetic storm. The low-latitude ionospheric perturbations in these events were similar to supersonic equatorial bubbles, triggered by the destabilizing effects of the upward ExB drift associated with the eastward electric field. return to List of Abstracts
J. C. Foster, S. Cummer and U. S. Inan
Millstone Hill incoherent scatter radar elevation scans across mid latitudes captured the ionospheric response to storm-induced electric field and precipitation-induced changes near the equatorward extent of the expanding auroral region during the early phases of the November 3-4, 1993 magnetic storm. A prompt, short-duration increase in the upward plasma velocity to > 100 m/s at 23:19 UT on November 3, 1993 signaled the onset of the storm-induced enhancement of the eastward electric field over Millstone Hill at 54° magnetic latitude. This resulted in an uplifting of the F region ionosphere above the site by ~80 km by 23:30 UT. Formation of a narrow ionospheric trough poleward of the Millstone site accompanied the auroral convection enhancement at somewhat later times while plasma-sheet precipitation produced the ionization at altitudes between 200 km and 300 km at L < 4 in the pre-midnight sector observed by the radar. Strong precipitation of energetic particles from the outer radiation belt was observed by SAMPEX at L=3.7, near the equatorward limit of the plasma-sheet precipitation observed by DMSP and Millstone Hill. Phase and amplitude perturbations of VLF signals propagating in the earth-ionosphere waveguide serve to localize the energetic radiation-belt precipitation near L=3.7 and provide accurate timing of storm-induced energetic precipitation, whose onset was at ~23:32 UT in the pre-midnight sector. A later enhancement of the eastward electric field at latitudes equatorward of Millstone Hill and the storm-induced trough led to a perturbation of the mid-latitude ionosphere to L < 2 and is the subject of a companion paper [Foster et al., 1997]. return to List of Abstracts
John C. Foster
Storm-Enhanced Density (SED) seen North of Millstone Hill (gif format)
Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ~750 m s-1 with a flux of 10E14 m-2 s-1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions. return to List of Abstracts
John C. Foster
Vertical profiles of ionospheric parameters during O+ outflow event (gif format)
Radar observations from Millstone Hill have identified many interesting phenomena associated with magnetosphere-ionosphere coupling processes at the equatorward edge of the auroral region. A number of important magnetospheric boundaries are found near the latitude of the radar facility (55° magnetic) and these result in the structure and dynamics which characterize the pre-midnight sub-auroral ionosphere. The equatorward extent of the plasma sheet particle population lies on field lines near the plasmapause and precipitation from the plasma sheet alters the ionospheric conductances, currents and fields. During disturbed conditions, an intense (>100 mV/m) polarization electric field can be set up when freshly-injected plasma sheet ions lie equatorward of the electrons and this drives the latitudinally-narrow polarization jet or Sub-Auroral Ion Drifts (SAID) which directly or indirectly result in a variety of ionospheric phenomena in the pre-midnight sector. Associated with the region of strongest convection, a deep, narrow F region trough forms while, equatorward of this, sunward advection of plasma from later local times and lower latitudes leads to the region of Storm-Enhanced Density (SED). The pre-midnight polarization jet produces strong frictional heating near the F peak resulting in an expansion of the topside and heavy ion outflow which populates the magnetosphere with ionospheric O+. Stable Auroral Red arcs (SAR arcs) occur within the narrow trough, coincident with the polarization jet electric fields and the elevated electron temperatures found there. return to List of Abstracts
J. C. Foster, M. J. Buonsanto, M. Mendillo, D. Nottingham, F. J. Rich and W. Denig
Radar/optical/satellite observations spanning SAR arc (gif format)
During the March 20-22, 1990, magnetic storm, Millstone Hill radar and DMSP satellite observations detailed the conditions surrounding the occurrence of a SAR arc which was observed continuously through an 8-hour interval from dusk till past midnight in the North American sector. All-sky imaging with a 630.0-nm imager continually monitored the two-dimensional position and magnitude of the SAR arc emission while radar scans and satellite overflights measured magnetospheric inputs and ionospheric response. The arc was colocated with a deep, narrow plasma trough and a region of enhanced westward plasma convection of similar width situated immediately equatorward of the low-latitude extent of plasma sheet particle precipitation. A region of low-energy ion precipitation was observed at the equatorward edge of the SAR arc during a period of spatial/temporal coincident satellite/radar observations near the Millstone Hill longitude. The width of the SAR arc and related phenomena was of the order of 2°, and the ~200-R emission was associated with an electron temperature of ~3500°K and a 10¥ reduction of plasma density at an altitude of 450 km. The best-fit model for the emission intensities of both the SAR arc and the background airglow suggests that either the electron temperature at the center of the SAR arc was somewhat higher than observed by the radar (~4000°K), or the neutral densities, [O2] and [O], were increased by factors of 2 and 4, respectively, with respect to the MSIS values. The ionospheric trough and a colocated region of enhanced sunward convection (500 - 1700 m s-1) were observed in conjugate hemispheres throughout the local time range 18 - 02 MLT. The convection feature seen in association with the SAR arc had many of the characteristics of a subauroral ion drifts (SAID) event; we report here the first long-duration observations of a colocated SAID/SAR arc event. A narrow ionospheric trough developed during the interval when the SAID velocity was >1000 m s-1 and was accompanied by a weak (100 R) 630.0-nm emission. As the velocity fell to ~700 m s-1, the density in the trough recovered somewhat, and the arc intensity rose to ~300 R above background. This brighter period of the SAR arc occurred within a fossil trough/SAID. We conclude that there is a close spatial and temporal association among these three types of subauroral low-altitude phenomena - the SAR arc, the SAID event, and the fossil (convection-related) trough - and that this is indicative of the interrelationship of the magnetospheric processes and boundaries which are involved in their formation. return to List of Abstracts
J. C. Foster, J. A. Klobuchar, V. E. Kunitsyn, E. D. Tereshchenko, E. S. Andreeva, M. J. Buonsanto, P. Fougere, J. M. Holt, B. Z. Khudukon, W. Pakula, T. D. Raymund
RATE ionospheric reconstruction with Millstone Hill Radar elevation scan. (gif format)
In order to intercompare various techniques used in reconstructing tomographic images, and to benchmark those results with direct observations obtained by the incoherent scatter technique, an experimental campaign and subsequent analysis program - the Russian-American Tomography Experiment (RATE) - were implemented in late 1993. Russian experiment teams from the Polar Geophysical Institute in Murmansk and Moscow State University joined with American investigators from the Phillips Laboratory and the Massachusetts Institute of Technology and an array of four receiving stations was set up in the northeastern United States and in eastern Canada to obtain data for the tomographic reconstructions. Phase-difference and total-phase tomographic reconstruction techniques have been employed and are intercompared. The spatial/altitude distribution of ionospheric electron content was observed by the MIT Millstone Hill incoherent scatter radar which scanned the ionosphere in a plane parallel to the satellite overflights. We present preliminary reconstructions of the ionospheric structure observed during a severe mid-latitude ionospheric storm which took place during the campaign. The drastic large-scale changes in the ionospheric structure which accompanied the November, 1993 storm were well-observed by the two diagnostic techniques. return to List of Abstracts
H.-C. Yeh, J. C. Foster, F. J. Rich, and W. Swider
Under disturbed geomagnetic conditions the latitudinal profile of the westward ion convection (equivalent to poleward electric field) observed with the Millstone Hill incoherent scatter radar at dusk, often exhibits a double peak (dual maxima). During the height of the February 8-9, 1986, magnetic storm the Millstone Hill radar was in the evening local time sector (1600-2200 MLT). Radar observations indicate that high speed (>1000 m s-1) westward ion flow penetrated deeply below 50° invariant latitude (Å) and persisted for 6 hours between 2100 UT on February 8 and 0300 UT on February 9. The double-peaked ion convection feature was pronounced throughout the period, and the separation in the dual maxima ranged from 4° to 10°. The latitude positions of the high-latitude ion drift peak and the convection reversal varied in unison. The low-latitude ion drift peak (~49°Inv or L=2.3) did not show significant universal time/magnetic local time (UT/MLT) variation in its latitude location but showed a decrease in magnitude during the initial recovery phase of the storm. Using simultaneous particle (30 eV - 30 keV) precipitation data from the DMSP F6 and F7 satellites, we find the high-latitude ion drift peak to coincide with the boundary plasma sheet/central plasma sheet transition in the high ionospheric conductivity (>15 mho) region. The low-latitude ion drift peak lay between the equatorward edges of the electron and soft (< 1 keV) ion precipitation in the low conductivity region (~1 mho). A comparison between the low-altitude observations and simultaneous ring current observations from the high-altitude AMPTE satellite further suggests that the low-latitude ion drift peak is closely related to the maximum of the O+ dominated ring current energy density in magnetic latitude. The low-latitude ion drift peak is the low-altitude signature of the electric field shielding effect associated with ring current penetration into the outer layer of the storm time plasmasphere. Unlike the transient and localized subauroral ion drifts under moderately disturbed conditions, the intense westward ion drifts developed in response to heavy ion ring current shielding during a great magnetic storm can decouple from the high-latitude electric field and penetrate to very low latitudes and persist for long durations in the dusk and early afternoon MLT sectors. These features confirm the active role of storm time ring current dynamics in generating the low-latitude extension of the magnetospheric electric field. return to List of Abstracts
J. C. Foster, D. Tetenbaum, C. F. Del Pozo, J.-P. St-Maurice, and D. R. Moorcroft
Aspect-Angle variation of coherent backscatter intensity (gif format)
The sensitivity of the Millstone 440-MHz radar system is such that coherent echoes from E region irregularities can be observed over a 90-dB dynamic range above the incoherent scatter background. At antenna elevation angles between 4° and 20°, aspect angles between 0° and 10° (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61°Å and 57°Å. During disturbed conditions, when convection electric fields in excess of 15 mV/m and E region irregularities span this range of latitudes, antenna scanning experiments have been performed to determine the aspect angle sensitivity with high precision. Our measurements are unique in that they provide a clear high-frequency description of the variation in both power and Doppler shift as functions of aspect angle, all the way from a region where the waves are known to be linearly unstable, in a direction perpendicular to the geomagnetic field, to as much as 10° away from perpendicularity. We find that the 440-MHz aspect sensitivity is about -15 dB deg-1 for aspect angles between 0° and 3°, -10 dB deg-1 for aspect angles between 3° and 6°, and -7 dB deg-1 for aspect angles between 6° and 9°. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angles <2° and decreases to <200 m/s as the aspect angle increases to >3°. For highly disturbed conditions the magnitude of the velocity can increase to >700 m/s for aspect angles <2°. The tendency for the altitude of the most intense return to decrease by ~5 km as the aspect angle increases beyond 2° can be explained as a consequence of the variation of aspect angle with height. return to List of Abstracts
J. C. Foster and D. Tetenbaum
A 40-µs pulse length has been used to provide 10-s temporal and 6-km range resolution observations of E region coherent backscatter from the premidnight eastward electrojet region to the north of Millstone Hill. The sensitivity of the Millstone UHF system is such that coherent returns can be observed over a 80-dB dynamic range and at levels down to the incoherent scatter background. Our observations can be divided into two categories: strong events in which the backscattered amplitude nears saturation and weak events in which spatial structure and large-amplitude variations are common. Calibrated observations find a typical volume scattering coefficient of ~10E-11m-1 at 440 MHz during strong events with a maximum level of 9*10E-10m-1 observed for brief intervals. During less intense events the radar backscatter is modulated by ~30 dB in amplitude at Pc 5 frequencies (150 - 500 s) by waves with spatial wavelength 50 - 100 km. Our observations support the premise that the weak irregularities grow linearly with electric field strength and reach a saturation amplitude beyond which the oscillating electric field of the Pc pulsation has little effect. The observed variation of backscattered power with range is interpreted using a geometrical model which accounts for the detailed antenna beam pattern, a magnetic aspect angle sensitivity of -10 dB per degree, and a thin layer of irregularities centered at 110 km altitude. For strongly driven conditions a comparison of the range variation of backscattered power with our thin layer model suggests that the signal power becomes increasingly dominated by strong scatterers confined to a narrower altitude range. The apparent altitude extent of the strongest irregularities decreases by a factor of 2 as the amplitude of the backscattered signal increases by a factor of 10. return to List of Abstracts
J. C. Foster and D. Tetenbaum
Phase velocity observations at E region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34-cm volume backscatter cross section, sigma, less than ~ 5*10E-13 m-1, the phase speed is at or below the usual ion acoustic speed in the E region (350 m/s), and increases only slowly with the observed backscattered power amplitude (~50 m/s per 10 dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (~200 m/s per 10 dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E region due to changing activity conditions or the effects of superimposed magnetospheric pulsations. return to List of Abstracts
http://www.haystack.edu/~jcf/papers.htm -- Revised: Nov 20, 1997
E-mail:
jcf@hyperion.haystack.edu
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