R. A. Heelis

Longitude variations in ion composition in the morning and evening topside equatorial ionosphere near solar minimum

Ion composition data from the Defense Meteorological Satellite Program (DMSP) F10 have been averaged by geographic longitude and dip latitude for the months of June, September, and December 1993. The data were taken under near solar minimum conditions. Near 800 km at two fixed local times near 0920 hours and 2120 hours, and at all longitudes, significant variation in local time and season are found. Longitude variations are consistent with modulation of the F peak height by meridional and zonal neutral winds. The components of these winds parallel to the magnetic field lines act to raise and lower the height of the F peak and, additionally, at night, to modulate the plasma decay rate. Zonal winds were found to have significant effects in the longitude regions 150°E to 270°E and 300°E to 360°E, where the magnetic declination is significant. Under solstice conditions, the summer to winter meridional winds play a dominant role in regulating the F peak height, with the zonal winds enhancing or opposing the effects of the meridional winds at longitudes with significant magnetic declination. Zonal winds dominate the regulation of the F peak height near equinox, when the meridional winds are fairly symmetric about the dip equator. The longitude variations are most clearly seen in the O+ and H+ concentrations when O+ is the dominant ion and is in equilibrium with H+. These conditions were found during the daytime during all seasons. H+ is frequently the dominant ion near 800 km, and at night, the longitudinal variations clearly seen in the O+ concentrations were not as easily seen in the H+ concentrations due to the larger scale height of H+.

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.

Ground and Space-Based Measurement of Rocket Engine Burns in the Ionosphere

On-orbit firings of both liquid and solid rocket motors provide localized disturbances to the plasma in the upper atmosphere. Large amounts of energy are deposited to ionosphere in the form of expanding exhaust vapors which change the composition and flow velocity. Charge exchange between the neutral exhaust molecules and the background ions (mainly O+) yields energetic ion beams. The rapidly moving pickup ions excite plasma instabilities and yield optical emissions after dissociative recombination with ambient electrons. Line-of-sight techniques for remote measurements rocket burn effects include direct observation of plume optical emissions with ground and satellite cameras, and plume scatter with UHF and higher frequency radars. Long range detection with HF radars is possible if the burns occur in the dense part of the ionosphere. The exhaust vapors initiate plasma turbulence in the ionosphere that can scatter HF radar waves launched from ground transmitters. Solid rocket motors provide particulates that become charged in the ionosphere and may excite dusty plasma instabilities. Hypersonic exhaust flow impacting the ionospheric plasma launches a low-frequency, electromagnetic pulse that is detectable using satellites with electric field booms. If the exhaust cloud itself passes over a satellite, in situ detectors measure increased ion-acoustic wave turbulence, enhanced neutral and plasma densities, elevated ion temperatures, and magnetic field perturbations. All of these techniques can be used for long range observations of plumes in the ionosphere. To demonstrate such long range measurements, several experiments were conducted by the Naval Research Laboratory including the Charged Aerosol Release Experiment, the Shuttle Ionospheric Modification with Pulsed Localized Exhaust experiments, and the Shuttle Exhaust Ionospheric Turbulence Experiments.

ROCSAT 1 ionospheric plasma and electrodynamics instrument observations of equatorial spread F: An early transitional scale result

Ion density and vertical ion drift velocity sampled at 1024 Hz from the ROCSAT 1 satellite are used to examine the behavior of horizontal structures in equatorial spread F near 600 km altitude. An initial investigation shows that at scale sizes less than 100 m the relationships between the vertical drift and density structure are distinguished by the bulk plasma flow in the structure and by the background gradient in the density. Two adjacent equatorial bubble structures are examined: one in which the bulk plasma flow is upward and characteristic of an active evolving bubble and the other in which the bulk plasma flow is small and characteristic of a stagnated structure. We find that at scale sizes less than 100 m the velocity structure has a spectral slope that is consistently shallower than that of the density structure in the stagnated bubble but matches closely the spectrum of the density structure in the active bubble. Each of the bubble regions examined are characterized by one edge that has a much larger background gradient than the other. We find that in these bubbles, observed near 600-km altitude, the lower background gradient is characterized by enhanced structure at 1-km scale sizes.

Storm time plasma irregularities in the pre-dawn hours observed by the low-latitude ROCSAT-1 satellite at 600 km altitude

Large scale ion density depletions were detected in the nighttime sector by ROCSAT-1 for over 10 hours during the 22 October 1999 geomagnetic storm. Prominent depletion structures (bubbles) that are characterized by large-amplitude density decrease (N/No ≤ 1%) with rapid horizontal ion drift (600 ∼ 800 m/s) are found to cluster in the 03:00 ∼ 04:30 local time sector and at magnetic latitudes 14° ∼ 20° S when the storm was in its early recovery phase. These presunrise bubbles are positively correlated to the enhanced eastward electric fields of greater than 1 ∼ 2 mV/m, which were in response to the storm-time disturbances resulting from the in-phase contributions of the prompt penetration magnetospheric and the long lasting ionospheric disturbance dynamo electric fields. Further analyses of the field-aligned and cross-field ion drifts within the depletions reveal that bubble plasma were driven by the eastward polarization electric fields to move upward, but these upward velocities were compensated by large downward field-aligned diffusive motions. These features confirm that the disturbance electric fields produced during a great magnetic storm can significantly affect the occurrence timing and spatial extent of severe plasma irregularities in low-latitude ionosphere. The spatial dimensions of the pre-sunrise irregularities may exceed the large region observed by the 35° inclined circular orbiting ROCSAT-1.

Storm time signatures of the ionospheric zonal ion drift at middle latitudes

[1] Using a quantitative identification of convection and particle boundaries at high latitudes, the relative extent of auroral convection and auroral precipitation is examined during superstorm events. During superstorms the convection reversal boundary normally located near 75° magnetic latitude moves to magnetic latitudes near 60°. The edge of the diffuse auroral precipitation that normally terminates near 60° magnetic latitude moves to magnetic latitudes near 40°. This limited study shows that during the main phase of the superstorm ion drifts driven by the magnetosphere penetrate to latitudes as low as the dip equator on the dusk side but extend only a few degrees equatorward of the auroral zone on the dawn side. Evidence for ion drifts driven by a disturbance dynamo may be found during the storm recovery phase when the interplanetary magnetic field is less strongly southward or turns northward, but the previously established flows below the auro?al region remain.

A Comparison of in situ measurements of E→ and −V→×B→ from Dynamics Explorer 2

Dynamics Explorer-2 provided the first opportunity to make a direct comparison of in situ measurements of the high-latitude convection electric field by two distinctly different techniques. The vector electric field instrument (VEFI) used antennae to measure the intrinsic electric fields and the ion drift meter (IDM) and retarding potential analyzer (RPA) measured the ion drift velocity vector, from which the convection electric field can be deduced. The data from three orbits having large electric fields at high latitude are presented, one at high, one at medium, and one at low altitudes. The general agreement between the two measurements of electric field is very good, with typical differences at high latitudes of the order of a few millivolts per meter, but there are some regions where the particle fluxes are extremely large (e.g., the cusp) and the disagreement is worse, probably because of IDM difficulties. The auroral zone potential patterns derived from the two devices are in excellent agreement for two of the cases, but not in the third, where bad attitude data may be the problem. At low latitudes there are persistent differences in the measurements of a few millivolts per meter, though these differences are quite constant from orbit to orbit. This problem seems to arise from some shortcoming in the VEFI measurements. Overall, however, these measurements confirm the concept of “frozen-in” plasma that drifts with velocity E→×B→/B2 within the measurement errors of the two techniques.

Seasonal and latitudinal distributions of the dominant light ions at 600 km topside ionosphere from 1999 to 2002

Data taken by the Republic of China satellite (ROCSAT-1) during moderate to high solar activity years from 1999 to 2002 have been studied for the statistical distribution of the dominant light ion species, either hydrogen or helium ions, at 600 km topside ionosphere. The results indicate some interesting seasonal and longitudinal/latitudinal distributions of the dominant light ions in the topside ionosphere during the magnetic quiet periods. Each light ion species can become the dominant ion species at 600 km topside ionosphere but only at night when the ion temperature is cooler than during the day. More cases of H + dominance have been observed than those of He + dominance. Except for the March equinox the distribution of dominant H + shows a strong hemispheric asymmetry for the other three seasons. When H + dominance is observed in one hemisphere during the solstice season, the low latitude limit of this transition region is a constant dip latitude in the winter hemisphere. This statistical minimum of the transition latitude shows little dependence on the seasonal averaged solar flux intensity. Similar hemispherically asymmetric distribution for dominant He + in the winter hemisphere during the solstice season has also been noted except that the asymmetrical pattern is not as prominent as in the dominant H + case because much fewer cases have been observed for dominant He + . The asymmetrical distribution of the dominant light ions seems to be related to the observed hemispheric field-aligned ion flow pattern. Thus it is concluded that the downward field-aligned ion flow together with the nighttime lower ion temperature in the winter hemisphere compose a possible cause for the occurrence distribution of the hemispheric asymmetry in the dominant light ion species. This can be understood from the fact that the field-aligned flow is related to the hemispheric asymmetry of the ionospheric F peaks and serves to enhance or retard the nocturnal redistribution of the light ions along the field line.

Vertical ExB drifts from radar and C/NOFS observations in the Indian and Indonesian sectors: Consistency of observations and model

In this paper, we analyze vertical ExB drifts obtained from the Doppler shifts of the daytime 150 km radar echoes from two radar stations located off the magnetic equator, namely, Gadanki in India and Kototabang in Indonesia, and compare those with corresponding Coupled Ion Neutral Dynamics Investigation (CINDI) observations onboard the C/NOFS satellite and the Scherliess-Fejer model in an effort to understand to what extent the low-latitude vertical ExB drifts of the 150 km region represent the F region vertical ExB drifts. The radar observations were made during 9–16 LT in January, June, July, and December 2009. A detailed comparison reveals that vertical ExB drifts observed by the radars at both locations agree well with those of CINDI and differ remarkably from those of the model. Importantly, the model and observed drifts show large disagreement when the observed drifts are either large or downward. Further, while the CINDI as well as the radar observations from the two longitudes are found to agree with each other on the average, they differ remarkably on several occasions when compared on a one-to-one basis. The observed difference in detail is due to measurements made in different volumes linked with latitudinal and/or longitudinal differences and underlines the role of neutral dynamics linked with tides and gravity waves in the two longitude sectors on the respective vertical ExB drifts. The results presented here are the first of their kind and are expected to have wider applications in furthering our understanding on fine-scale longitudinal variabilities in the ionosphere in general and ionospheric electrodynamics in the Indian and Indonesian sectors in particular.

Response of the ionospheric convection reversal boundary at high latitudes to changes in the interplanetary magnetic field

In this paper we present a systematic study of the location of the convection reversal boundary for southward interplanetary magnetic field by using Defense Meteorological Satellite Program (DMSP) F13 and F15 spacecraft measurements during local summer seasons from 2000 to 2007 for both hemispheres. All the convection reversal boundaries are identified pass bypass by locating the highest-latitude location where the plasma flow shows a large-scale two-cell pattern and reverses direction from sunward to antisunward. The location of the convection reversal boundaries are placed into 10 different categories based on By and the magnitude of southward Bz. Observations suggest that (1) the location of the boundary is well organized by the magnitude of Bz, being at lower latitudes for stronger negative Bz and also organized by the polarity of By, moving toward the dawnside/duskside when By changes from negative to positive in the northern/southern hemisphere; (2) the average latitudinal movement of the boundary associated with By changes is comparable to the average movement of the boundary with Bz changes; (3) an initial reconfiguration of the boundary near local noon is redistributed around the dawnside or duskside dependent on the direction of By; and (4) the boundary has a general spiral shape, which varies depending on Bz and By.