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1.
The rates of heat input into the mesosphere and lower thermosphere are calculated and compared with the heat losses. The worldwide average eddy diffusion coefficient required to maintain continuity in the heat budget is calculated and found to vary from about 107 cm2/sec at 120 km down to about 105 cm2/sec at 60 km. From the global asymmetry in heating at the solstice, it is concluded that a systematic pattern of vertical velocities prevails ranging from less than 1 cm/sec in the mesosphere up to 10 cm/sec near 120 km, upward over the summer polar region and downward over the winter polar region. This can be balanced by a wind system towards the winter polar region with velocities near 1 m/sec at 60 km increasing to 30 m/sec at 120 km. Such a wind system provides an explanation for the helium bulge in the upper thermosphere over the winter polar region.  相似文献   

2.
The distribution of stably trapped plasma is determined as a function of equatorial geocentric distance, R. Observed Pi2 micropulsation periods and the magnitudes of their associated magnetic bays, measured by the auroral electrojet index, AE, are used in determining this distribution. Plasma density is found to vary inversely as approximately the sixth power of R. When Pi2 period-AE relations are studied for varying time lags, the pulsation periods are found to define prolonged intervals of abnormal AE levels. Thus the periods may be potentially useful in forecasting geomagnetic disturbances or abnormally quiet intervals. A qualitative physical model is proposed to explain the observational relations.  相似文献   

3.
The theory of dissipation of ionospheric electric currents is extended to include viscosity. In a steady state (i.e. usually above about 140 km altitude) the joule plus viscous heating may be calculated by μ∇2v. E × B/B2. At lower altitudes where viscosity may, in some circumstances, be relatively unimportant the joule dissipation is calculated by the usual formula j. (E + v × B). In a prevalent model of the auroral electrojets it is found that the joule heating can be much more intense outside auroral forms than within them. Heating due to auroral electrojets cause a semi-annual variation in the thermosphere. Movement caused by auroral electric fields make a contribution to the super-rotation of the midlatitude upper atmosphere. Random electric fields lead to an eddy ‘viscosity’ or ‘exchange coefficientrs in the upper thermosphere of magnitude ρER2/B3tR2|∇E|. where tR is the correlation time of the random component of electric fields ER and ρ is air density. Theoretical conditions for significant heating by field-aligned currents are derived.  相似文献   

4.
During the period October to December 1981, the Dynamics Explorer-2 (DE-2) spacecraft successively observed the South polar and the North polar regions, and recorded the temperature, composition and dynamical structure of the upper thermosphere. In October 1981, perigee was about 310 km altitude, in the vicinity of the South Pole, with the satellite orbit in the 09.00–21.00 L.T. plane. During late November and December, the perigee had precessed to the region of the North Pole, with the spacecraft sampling the upper thermosphere in the 06.00 18.00 L.T. plane. DE-2 observed the meridional wind with a Fabry-Perot interferometer (FPI), the zonal wind with the wind and temperature spectrometer (WATS), the neutral temperature with the FPI, and the neutral atmosphere composition and density with the neutral atmosphere composition spectrometer (NACS). A comparison between the South (summer) Pole and the North (winter) Pole data shows considerable seasonal differences in all neutral atmosphere parameters. The region of the summer pole, under similar geomagnetic and solar activity conditions, and at a level of about 300 km, is about 300 K warmer than that of the winter pole, and the density of atomic oxygen is strongly depleted (and nitrogen enhanced) around the summer pole (compared with the winter pole). Only part of the differences in temperature and composition structure can be related to the seasonal variation of solar insolation, however, and both polar regions display structural variations (with latitude and Universal Time) which are unmistakeable characteristics of strong magnetospheric forcing. The magnitude of the neutral atmosphere perturbations in winds, temperature, density and composition within both summer and winter polar regions all increase with increasing levels of geomagnetic activity.The UCL 3-dimensional time dependent global model has been used to simulate the diurnal, seasonal and geomagnetic response of the neutral thermosphere, attempting to follow the major features of the solar and geomagnetic inputs to the thermosphere which were present during the late 1981 period.In the UCL model, geomagnetic forcing is characterized by semi-empirical models of the polar electric field which show a dependence on the Y component of the Interplanetary Magnetic Field, due to Heppner and Maynard (1983), It is possible to obtain an overall agreement, in both summer and winter hemispheres, with the thermospheric wind structure at high latitudes, and to explain the geomagnetic control of the combined thermal and compositional structure both qualitatively and quantitatively. To obtain such agreement, however, it is essential to enhance the polar ionosphere as a consequence of magnetospheric particle precipitation, reflecting both widespread auroral (kilovolt) electrons, and “soft” cusp and polar cap sources. Geomagnetic forcing of the high latitude thermosphere cannot be explained purely by a polar convective electric field, and the thermal as well as ionising properties of these polar and auroral electron sources are crucial components of the total geomagnetic input.  相似文献   

5.
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6.
A previous comparison of experimental measurements of thermospheric winds with simulations using a global self-consistent three-dimensional time-dependent model confirmed a necessity for a high latitude source of energy and momentum acting in addition to solar u.v. and e.u.v. heating. During quiet geomagnetic conditions, the convective electric field over the polar cap and auroral oval seemed able to provide adequate momentum input to explain the thermospheric wind distribution observed in these locations. However, it seems unable to provide adequate heating, by the Joule mechanism, to complete the energy budget of the thermosphere and, more importantly, unable to provide the high latitude input required to explain mean meridional winds at mid-latitudes. In this paper we examine the effects of low energy particle precipitation on thermospheric dynamics and energy budget. Modest fluxes over the polar cap and auroral oval, of the order of 0.4 erg cm −2/s, are consistent with satellite observations of the particles themselves and with photometer observations of the OI and OII airglow emissions. Such particle fluxes, originating in the dayside magnetosheath cusp region and in the nightside central plasma sheet, heat the thermosphere and modify mean meridional winds at mid-latitudes without enhancing the OI 557.7 line, or the ionization of the lower thermosphere (and thus enhancing the auroral electrojets), neither of which would be consistent with observations during quiet geomagnetic conditions.  相似文献   

7.
We study the propagation of gravity waves in the martian atmosphere using a linearized one-dimensional full-wave model. Calculations are carried out for atmospheric parameters characteristic of Mars Orbiter Laser Altimeter (on Mars Global Surveyor MGS) observations of apparent gravity waves in high latitude clouds and MGS radio occultation measurements of temperature variations with height suggestive of gravity wave activity. Waves that reach the thermosphere produce fluctuations in density comparable in amplitude with the density variations detected in Mars Odyssey aerobraking data. Gravity waves of modest amplitude are found to deposit momentum and generate significant heating and cooling in the martian atmosphere. The largest heating and cooling effects occur in the thermosphere, at altitudes between about 130 and 150 km, with heating occurring at the lower altitudes and cooling taking place above.  相似文献   

8.
Using a three-dimensional, time-dependent, global model, we have simulated the response of the thermosphere to an isolated substorm. The substorm is characterized by a time variance of the high latitude convective electric field with an associated enhancement of auroral E region electron density, from an initially quiet thermosphere. We have simulated such an impulsive energy input with both separated and co-incident geographic and geomagnetic poles and have found that, in both cases, in the lower thermosphere ( ～ 120 km), a long-lived vortex phenomenon is generated. Initially, two contra-rotating vortices are generated by the effects of ion drag during the period of enhanced high latitude energy input centred on the polar cap/auroral oval boundary, one at dusk (18.00 L.T.) and the other at dawn (06.00 L.T.). After the end of the substorm, the cyclonic vortex (dawn) dissipates rapidly while the dusk anti-cyclonic vortex appears virtually self-sustaining and survives many hours after the substorm input has ceased. A theory is derived to explain and interpret the results and it appears that the effect is analogous to a meteorological weather system. In this case, however, the dusk anti-cyclonic vortex has, instead of pressure, the centrifugal acceleration balancing the Coriolis force. The equivalent anti-clockwise dawn vortex, unlike a low pressure system, has no balancing force, since Coriolis and the centrifugal term assist and this vortex rapidly disappears.  相似文献   

9.
One of the most consistent and often dramatic interactions between the high latitude ionosphere and the thermosphere occurs in the vicinity of the auroral oval in the afternoon and evening period. Ionospheric ions, convected sunward by the influence of the magnetospheric electric field, create a sunward jet-stream in the thermosphere, where wind speeds of up to 1 km s?1 can occur. This jet-stream is nearly always present in the middle and upper thermosphere (above 200 km altitude), even during periods of very low geomagnetic activity. However, the magnitude of the winds in the jet-stream, as well as its location and range in latitude, each depend on geomagnetic activity. On two occasions, jet-streams of extreme magnitude have been studied using simultaneous ground-based and satellite observations, probing both the latitudinal structure and the local time dependence. The observations have then been evaluated with the aid of simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics including the effects of magnetospheric convection and particle precipitation. The extreme events, where sunward winds of above 800 ms?1 are generated at relatively low geomagnetic latitudes (60–70°) require a greatly expanded auroral oval and large cross-polar cap electric field ( ~ 150 kV). These in turn are generated by a persistent strong Interplanetary Magnetic Field, with a large southward component. Global indices such as Kp are a relatively poor indicator of the magnitude and extent of the jet-stream winds.  相似文献   

10.
The global distribution and temporal variations of thermospheric heating due to Joule dissipation of measured ionospheric electric fields are computed. It is shown that the volume Joule dissipation rate at high and middle latitude is similar in magnitude and altitudinal profile to the global solar EUV absorption rate discussed in the previous papers. Thus, Joule dissipation contributes significantly towards reconciling the quantitatively known sources of thermospheric heat input and that required to maintain the normal thermosphere. The combined heat source due to EUV absorption and Joule dissipation varies with the annual cycle in a manner closely resembling that of the thermospheric density.  相似文献   

11.
In the first few tens of minutes after the onset of widespread Joule heating, the motion of the ionospheric atmosphere can be approximated by the one-dimensional motion of a gas in a gravity field—a problem that is easily solved because the motion takes place at constant pressure. The solution provides an estimate of time for which the model is applicable to the physical situation. Seasonal variations of the early effects are examined by using ion profiles appropriate to each season. The results show that the atmosphere above 100 km is strongly modified within a few tens of minutes after the onset of widespread heating: the density can double, the temperature can increase several hundred degrees, and the molecular nitrogen concentration can quadruple. Vertical winds exceeding 100 m/sec at 400km altitude are possible for a brief period after the onset of electric fields of 100 mV/m—rare but observed events. In the first few tens of minutes after the onset of a given electric field, the greatest power is deposited in the thermosphere around summer solstice, while the greatest winds occur at 200 km altitude in the summer and at 400km in the winter. These differing seasonal effects show primarily that a given level of change occurs sooner for one season than another, not that long term seasonal differences exist. Once a magnetic storm is in progress, the quiet-day ion profiles change to the non-seasonal storm profile ; for this ion distribution, F-region effects are minimum regardless of season. Joule heating effects in the upper thermosphere are therefore concluded to be self-limiting.  相似文献   

12.
We present an analysis of a series of observations of the auroral/polar regions of Jupiter, carried out between September 8 and 11, 1998, making use of the high-resolution spectrometer, CSHELL, on the NASA InfraRed Telescope Facility (IRTF), Mauna Kea, Hawaii; these observations spanned an “auroral heating event.” This analysis combines the measured line intensities and ion velocities with a one-dimensional model vertical profile of the jovian thermosphere/ionosphere. We compute the model line intensities both assuming local thermodynamic equilibrium (LTE) and, relaxing this condition (non-LTE), through detailed balance calculations, in order to compare with the observations. Taking the model parameters derived, we calculate the changes in heating rate required to account for the modelled temperature profiles that are consistent with the measured line intensities. We compute the electron precipitation rates required to give the modelled ion densities that are consistent with the measured line intensities, and derive the corresponding Pedersen conductivities. We compute the changes in heating due to Joule heating and ion drag derived from the measured ion velocities, and modelled conductivities, making use of ion-neutral coupling coefficients derived from a 3-D global circulation model. Finally, we compute the cooling due to the downward conduction of heat and the radiation-to-space from the molecular ion and hydrocarbons. Comparison of the various heating and cooling terms enables us to investigate the balance of energy inputs into the auroral/polar atmosphere. Increases in Joule heating and ion drag are sufficient to explain the observed heating of the atmosphere; increased particle precipitation makes only a minor heating contribution. But local cooling effects—predominantly radiation-to-space—are shown to be too inefficient to allow the atmosphere to relax back to pre-event thermal conditions. Thus we conclude that this event provides observational, i.e. empirical, evidence that heat must be transported away from the auroral/polar regions by thermally or mechanically driven winds.  相似文献   

13.
The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCAR's thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F-region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E-region ionosphere.

The results of several TGCM runs are used to show that at F-region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid.

The F-region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer (DE−2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s−1) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s−1). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE−2 satellite measurements.  相似文献   


14.
We present the first 3-dimensional self-consistent calculations of the response of Saturn's global thermosphere to different sources of external heating, giving local time and latitudinal changes of temperatures, winds and composition at equinox and solstice. Our calculations confirm the well-known finding that solar EUV heating alone is insufficient to produce Saturn's observed low latitude thermospheric temperatures of 420 K. We therefore carry out a sensitivity study to investigate the thermosphere's response to two additional external sources of energy, (1) auroral Joule heating and (2) empirical wave heating in the lower thermosphere. Solar EUV heating alone produces horizontal temperature variations of below 20 K, which drive horizontal winds of less than 20 m/s and negligible horizontal changes in composition. In contrast, Joule heating produces a strong dynamical response with westward winds comparable to the sound speed on Saturn. Joule heating alone, at a total rate of 9.8 TW, raises polar temperatures to around 1200 K, but values equatorward of 30° latitude, where observations were made, remain below 200 K due to inefficient meridional energy transport in a fast rotating atmosphere. The primarily zonal wind flow driven by strong Coriolis forces implies that energy from high latitudes is transported equatorward mainly by vertical winds through adiabatic processes, and an additional 0.29-0.44 mW/m2 thermal energy are needed at low latitudes to obtain the observed temperature values. Strong upwelling increases the H2 abundances at high latitudes, which in turn affects the H+3 densities. Downwelling at low latitudes helps increase atomic hydrogen abundances there.  相似文献   

15.
Neutral density data were obtained near 400km (1600 LT) from a microphone density gauge on OGO-6 from 0°G to 40°N magnetic latitude for 25 September–3 October 1969. Several geomagnetic storms occurred during this period (ap varied from 0 to 207). Least-squares fits were made to data points on density-ap and density-Dst scatter diagrams, where the density values selected were delayed in time behind ap and Dst. An equation representing the least-squares fit was computed for each delay time. The equation of best fit (and the corresponding time delay between the density and the magnetic index which resulted in this best fit) was found by choosing the equation that gave the minimum standard error. For example, the best fit at 10°N geomagnetic latitude occurred for ap at t — 3 hr, where t is the time of the density values. The implications of the time differences associated with the best fits at various latitudes and longitudes are discussed with regard to the time delays involved in geomagnetic heating of the neutral upper atmosphere.

A low-latitude density bulge has been found between 0°N and 40°N whose magnitude varies with ap. DeVries (1972b) has independently discovered this daytime phenomenon. If the bulge is a semi-permanent feature near the equinoxes because of the enhanced geomagnetic activity, this may help explain the semi-annual effect in density, which was uncovered first in the drag data from low inclination satellites.  相似文献   


16.
Auroral E region neutral winds determined from incoherent scatter radar observations at Chatanika, AK, during geomagnetic disturbances (15 May 1974) are compared with detailed theoretical calculations of neutral velocities for these conditions. The theoretical velocities are obtained by numerically solving the ion and neutral momentum equations in the ion drag approximation, including coriolis and viscous forces, using observed electric fields and electron densities. Large vertical gradients are found in the calculated velocities for altitudes below about 130 km. As a consequence of this structure and fluctuations in the electron density profiles, the data analysis procedure of Brekke et al. (1973) for obtaining neutral winds from radar data is found to underestimate the wind speed by up to 40%, but it determines the direction and temporal structure reasonably well. Comparison of observed neutral velocities with calculated values shows that ion drag alone cannot account for the observations. An equation is derived to estimate the pressure gradients required to resolve the discrepancy between calculated and observed neutral winds. Accelerations due to these pressure gradients are of the same order as those due to ion drag, but at least an order of magnitude larger than those due to solar heating. Directions of the horizontal pressure gradients are consistent with expected locations of auroral heating. During geomagnetic disturbances, ion drag and auroral heating both appear to play important roles in the generation and modification of neutral winds.  相似文献   

17.
Raeder  J.  Wang  Y.L.  Fuller-Rowell  T.J.  Singer  H.J. 《Solar physics》2001,204(1-2):323-337
We present results from a global simulation of the interaction of the solar wind with Earth's magnetosphere, ionosphere, and thermosphere for the Bastille Day geomagnetic storm and compare the results with data. We find that during this event the magnetosphere becomes extremely compressed and eroded, causing 3 geosynchronous GOES satellites to enter the magnetosheath for an extended time period. At its extreme, the magnetopause moves at local noon as close as 4.9 R E to Earth which is interpreted as the consequence of the combined action of enhanced dynamic pressure and strong dayside reconnection due to the strong southward interplanetary magnetic field component B z, which at one time reaches a value of −60 nT. The lobes bulge sunward and shield the dayside reconnection region, thereby limiting the reconnection rate and thus the cross polar cap potential. Modeled ground magnetic perturbations are compared with data from 37 sub-auroral, auroral, and polar cap magnetometer stations. While the model can not yet predict the perturbations and fluctuations at individual ground stations, its predictions of the fluctuation spectrum in the 0–3 mHz range for the sub-auroral and high-latitude regions are remarkably good. However, at auroral latitudes (63° to 70° magnetic latitude) the predicted fluctuations are slightly too high. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014228230714  相似文献   

18.
The development of the auroral bulge during substorms is studied using all-sky data from the dense net of stations and also riometer data. A few features seem to be essential for the interpretation of the expansive phase mechanism. The first is the existence of low energy electron precipitation (auroral arcs with the lower border height near 140 km) polewards of the expanding bulge, suggesting that the bulge often arises and develops on closed field lines. Secondly only the localized bulges (with dimensions 2° and 40°Λ) are generated by the continuous deformation of the auroral arc. The greater expansions develop mainly at the expense of the new bright arc formations at the front of the expanding auroral bulge. During each new arc formation impulsive acceleration and precipitation of energetic electrons takes place and brief changes of plasma sheet geometry are sometimes observed at 18 RE in the magnetotail. This apparently shows a re-distribution of plasma sheet current during the substorm expansive phase.  相似文献   

19.
Excitation rates of the infrared emissions which are likely to occur in the mesosphere and thermosphere are quantitatively evaluated. They include the 9.6 μm band of O3, the 15 and 4.3 μm bands of CO2 and the 5.3 and 2.8 μm bands of NO. These emissions may be excited through nonthermal processes such as chemiluminescent reactions and resonant fluorescence in the thermosphere, whereas they are of thermal origin in the stratosphere and mesosphere. Increase of the non-thermal excitation rate caused by precipitating electrons could be responsible for the enhancement of the 4.3 μm band of CO2, and the 5.3 and 2.8 μm bands of NO observed in the auroral thermosphere.  相似文献   

20.
The photodissociation of oxygen in the lower thermosphere is evaluated to obtain its global average value and the hemispheric imbalance. The observed concentrations of atomic oxygen do not reflect this imbalance in production due to the effect of seasonal wind patterns redistributing the atomic oxygen. The wind system necessary to compensate for the imbalance in solar thermal input into the lower thermosphere is found to transport an amount of atomic oxygen sufficient to compensate for the hemispheric imbalance in production. Ionospheric data indicate a winter enhancement in atomic oxygen concentration; to produce this, a higher degree of oxygen dissociation than that normally accepted (i.e. higher than an atomic to molecular oxygen ratio of unity at 120 km) is needed. The concept that the concentrations of atomic oxygen observed over the winter polar region are maintained by transport from lower latitudes requires that eddy diffusion coefficients derived from vertical transport at low latitudes (ignoring horizontal transport) be reduced by about 25 per cent.  相似文献   

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