Neutral Density Enhancements Research Articles

Extreme Poynting flux in the dayside thermosphere: Examples and statistics

[1] With the launch of the Defense Meteorological Satellite Program F-15 spacecraft in late 1999, data for calculating Earth-directed, magnetospheric Poynting flux became available for the 09–21 solar local time sectors. We have assembled a data base for this key element of the upper atmosphere energy budget, for the interval 2000–2005. Here we briefly introduce the data set and show a subset that reveals a pattern of extreme Poynting flux deposition associated with a large east-west interplanetary magnetic field component. At such times the dayside high-latitude Poynting flux may exceed 170 mW/m2—an order of magnitude above typical values. The likely source of these events is merging at the magnetopause flank and lobe. A significant fraction of these events occur with high speed solar wind. This pattern of extreme Poynting flux deposition has, to date, eluded detection. Energy deposition at these high rates is a likely source of previously reported, but poorly understood, near-cusp neutral density enhancements.

Thermospheric response to ion heating in the dayside cusp

Recent observations from the CHAMP satellite indicate that neutral density enhancements are common in the northern dayside cusp. The neutral density in this region can be nearly a factor of 2 larger than in adjacent regions of the thermosphere on the poleward and equatorward sides of the cusp. The presence of density enhancements implies that the neutral atmosphere is being heated in the cusp region causing upwelling. A high-resolution model of the global thermosphere is used to study the thermospheric response to heating in the northern dayside cusp. It is found that heating in the cusp results in the creation of a neutral fountain. Specifically, upward drift of the thermosphere within the cusp region is followed at higher altitudes by poleward and equatorward movement out of the cusp region and the gradual subsidence of the neutral gas. Density enhancements of the magnitude observed by the CHAMP satellite occur in the model results for sufficiently strong heating in the cusp. Neutral temperature enhancements also occur and are strongest near the poleward and equatorward boundaries of the cusp region.

Local Neutral Density Enhancement by Streaming Neutral Gas Injection

An array of nozzles arranged in a bicycle-wheel spoke pattern designed to create a region of high neutral gas density in an H- ion source was tested for its capability to generate a pressure gradient in a vacuum chamber. An experimental setup was designed and built to measure the local pressure around the nozzle array. The two-dimensional direct simulation Monte Carlo (DSMC) technique was employed to simulate the local pressure distribution around the nozzles and to compare the results with the measurements. Pressure measurements and DSMC simulations showed that the streaming neutral gas injection (SNGI) technique was capable of increasing the local pressure to 200% when the ambient pressure was 3.0 ×10-2 Pa. At higher ambient pressures, the enhancement factor decreased to 58% when the ambient was 2.8 Pa. A sloshing-type SNGI was also designed and the highest enhancement was 87% at an ambient pressure of 2.6 ×10-1 Pa.

Effect of equatorial plasma bubbles on the thermosphere

Equatorial plasma bubbles are common in the low‐latitude ionosphere at night, particularly at solar maximum. The bubbles form on the bottomside of the F‐layer as a result of the Rayleigh‐Taylor instability and then drift upwards and to the east. As the bubbles evolve, the entire north‐south extent of the plasma flux tubes in the bubbles becomes depleted, and the bubbles take the form of vertically elongated wedges of depleted plasma. The east‐west width of a bubble domain can be several thousand kilometers and the plasma density depletion in the bubbles varies from a factor of 10 to 1000. Because equatorial plasma bubbles could have an appreciable effect on the upper atmosphere, a time‐dependent, three‐dimensional, high‐resolution model of the global thermosphere was used to calculate the response of the neutral gas to “idealized” plasma bubble depletions. The model predicts that there are both neutral density and temperature depressions and enhancements in association with the plasma bubbles. The bubble regions can contain either neutral gas enhancements or depressions depending on the background conditions, which change throughout the night. However, the calculated neutral gas perturbations are small, with maximum neutral density perturbations of 6% and maximum temperature perturbations of about 35°K. Nevertheless, these results support the recent experimental evidence that plasma bubbles produce depletions in the neutral density.

The effects of multiple propagating plasma patches on the polar thermosphere

Polar cap patches are the largest plasma structures in the high-latitude ionosphere. They have horizontal dimensions of hundreds to thousands of kilometers. The plasma density in the patches is similar to the density on the dayside, which is up to an order of magnitude higher than the local background density in the polar cap. Frequently, multiple propagating plasma patches are observed, which are elongated in a direction perpendicular to the generally antisunward motion. Because of the large-scale size, the presence and motion of polar cap patches may have a significant effect on the thermosphere. Previous studies of single patches have shown that the effect of a polar cap patch is significant. The direct effect on the neutral density is a snowplow effect. A plasma patch steepens the neutral density gradient in front of the patch and decreases the neutral density in and behind the patch. This effect is realized via a localized increase of the wind velocity. In the patch occupied region, the neutral temperature also increases, which creates a propagating neutral hot spot. In the present work, we used a time-dependent, 3-D thermospheric circulation model, with a high spatial resolution, to study the effects of multiple propagating patches on the thermosphere. A sequence of plasma patches, with horizontal dimensions of 200 km×2000 km , were simulated for different solar conditions and different patch-to-background density ratios. From these simulations, we found that both individual and collective effects of the patches on the thermosphere are evident. The most general effects are a neutral density depletion and heating. The density perturbations can be as large as 30%, and the temperature increases can reach about 400 K depending on the conditions. The individual effects show up as localized neutral density depletions and heating associated with each patch. Structures are created in the neutral density and temperature distributions. A sequence of patches also acts as one large structure. The neutral density enhancement only occurs in front of the first patch. The neutral density depletion and heating also occur in a large region outside the patch occupied regions, particularly in the front. From a systematic study with different conditions, we found that increasing the patch-to-background density ratio and the cross-cap potential both act to enhance the strength of the perturbation, although it is not in a linear proportion. The neutral density and temperature changes in absolute values, due to the plasma patches, are higher at solar maximum than at solar minimum. However, the relative changes (percentage) show the opposite trend. The perturbations in the neutral thermosphere penetrate to lower altitudes, and there is usually a time delay associated with this low-altitude penetration. It should be pointed out that representative patches are used in the simulations and the feedback from the modification of the thermosphere is not included. This theoretical study is not intended for a direct comparison to particular observations. Instead, it is to understand the physical process and to predict the result for future measurements.

Effect of polar cap patches on the thermosphere for different solar activity levels

Abstract Polar cap patches are regions of enhanced ionization that appear when the interplanetary magnetic field is southward. They are created either in or near the cusp and then convect in an antisunward direction across the dark polar cap at speeds of from 100 m/s to about 2 km/s. The size of these plasma patches varies from about 100 to 1000 km, and their density varies from 10 percent to over an order of magnitude relative to background ionospheric densities. To determine their effect on the thermosphere, a time-dependent global circulation model was used to simulate the thermospheric response to a plasma patch for both solar maximum and minimum conditions. The simulations indicate that for both solar maximum and minimum a propagating plasma patch acts as a collisional snowplow, creating a hole in the thermosphere in and behind the patch and a density enhancement at the front of the patch. The neutral temperature and wind are also increased. For plasma patches of similar strength (same horizontal and vertical dimensions and peak-to-background density ratio), there is a stronger and more localized thermospheric disturbance at solar minimum than at solar maximum. In fact, the neutral density enhancement at the front of a moderate patch is negligibly small at solar maximum even though the density depletion is still evident. These features can be explained by the lower neutral pressure and lower collision frequency in the neutral gas at solar minimum compared to solar maximum.

Pickup ions in the unshocked solar wind at comet Halley

The Tünde‐M experiment on board the VEGA 1 spacecraft detected energetic cometary ions as far as 10 million kilometers from comet Halley. The measured ion fluxes increased with decreasing radial distance from the nucleus, in agreement with theoretical expectations. Sharp enhancements spaced at approximately 4‐hour intervals were superimposed on the general increase of the ion fluxes on the inbound leg of VEGA 1. Periodic neutral structures originating from gas production by active areas on the nucleus are suggested to explain the recurrent intensity peaks. These neutral density enhancements would then produce the ion density enhancements seen by Tünde‐M as the spacecraft traversed the structures. This explanation requires a considerably larger neutral expansion velocity (≈6 km/s) than was actually measured near the nucleus in order to allow the rotation period of the nucleus to generate the observed period of the enhancements. No obvious correlations of the ion fluxes with the limiting energy for pickup were found. However, at a distance of about 107 km from the comet, a weak anticorrelation was found between the ion flux and the angle between the interplanetary magnetic field and solar wind direction. Cometary ion distribution functions in the solar wind reference frame were derived from measured counting rates. These distributions are approximately Maxwellian for energies between about 100 keV and 150 keV. The derived temperatures are between about 5 and 8 keV for most cometocentric distances, although in a region between 10 and 8 million km from the nucleus the ion spectra were harder, with temperatures ranging from 10 to 20 keV.

Charge exchange measurements of MHD activity during neutral beam injection in the Princeton Large Torus and the Poloidal Divertor Experiment

The horizontally scanning, multi-angle charge exchange analysers on the Princeton Large Torus (PLT) and the Poloidal Divertor Experiment (PDX) were used to study the effects of MHD activity on the background ion distribution function and on the beam ion slowing-down process during high power neutral injection. Sawtooth oscillations were observed in the fast ion flux on PLT and PDX, and measurements with neutral beams providing local neutral density enhancement indicated that the ions were transported radially when these events occurred. With nearperpendicular injection in PDX, at the lower toroidal fields necessary to maximize the plasma beta, repetitive bursts of greatly enhanced charge exchange flux were observed. These were associated with the ‘fishbone’ MHD instability, and a substantial depletion of the perpendicular slowing-down spectrum below the injection energy was seen. A simple phenomenological model for this loss mechanism was developed, and its use in simulation codes has been successful in providing good agreement with the experimental data. The behaviour and characteristics of this model are well matched by direct theoretical calculations.

Collision frequency measurements in the high‐latitude E region with EISCAT

Measurements of the ion‐neutral collision frequencies with the EISCAT UHF incoherent scatter radar in the high‐latitude, lower E region (90–105 km) show that for a time period of 2 weeks (at the beginning of February 1984) under different geophysical conditions the collision frequencies change little and compare well with a model calculated from the CIRA (1972) neutral atmosphere. When the ion and electron temperatures are approximately equal, it is confirmed that the measured collision frequency is consistent with the induced dipole collision frequency and an atmosphere in static equilibrium. One event at another time where collision frequencies increase by a factor of 1.5–2 over a period of 1½ hr could be due to neutral density enhancements from the passage of a large‐amplitude, long‐period gravity wave.