Ion and neutral composition changes in the thermospheric region during magnetic storms

The structural differences of the ion and neutral composition in the thermospheric region are studied by solving a system of basic ionospheric and atmospheric equations. The study shows that the compositional changes during a magnetic storm arise largely as a result of changes in the neutral composition at the turbopause. A decrease in [O]/[N₂] in the lower atmosphere triggers a complex chain of events which results in an increase of the neutral gas temperature, depletion of the O⁺ layer and enhancement of NO⁺. The relative changes in these layers occasionally produce a sequence of electron density profiles giving rise to the so-called G condition. It is shown that, compared to the neutral atmosphere, the ionosphere is much more sensitive to the changes in [O]/[N₂] in the lower thernaospheric region. Since the ionospheric parameters can be measured much more accurately than the atmospheric parameters, it is argued that they should form an integral part of the observational data required to construct the atmospheric models.     

A model of thermospheric temperature and composition

An empirical model of thermospheric temperature (T_∞T₁₂₀, and s) and composition (H, He, N, O, N₂, O₂, and Ar) was derived from measurements of 8 satellites (AE-C, AE-E, AEROS-A, AEROS-B, ARIEL-3, ESRO-4, OGO-6, and SAN MARCO-3) and 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, and St Santin). The altitude covered extends from 120 km up to about 600 km over the time period 1967 to 1976. The analytical framework used in the model resembles closely the MSIS setup: time independent terms, solar flux terms, geomagnetic activity (K_p) effect, annual (semiannual) and diurnal (semidiurnal, terdiurnal) variations, longitudinal terms, the U.T. effect, and corrections compensating for deviations from diffusive equilibrium at altitudes below 200 km. The model describes quiet to medium disturbed geomagnetic conditions (K_p ? 4) at solar fluxes (10.7cm) ranging from 60 to 180 × 10^?22 Wm^?2Hz^?1. To get an impression of the accuracy presently obtained, the model is compared with MSIS, Jacchia (1977), and the models of Thuillier (T_∞ and Engebretson (N). The best agreement is found for the temperature and the constituents He, O, and N₂ with increasing deviations in the order of H, N, Ar, and O₂.     

Neutral composition changes during a period of increasing magnetic activity

Neutral composition data obtained by the gas analyzer aboard the satellite ESRO 4 are investigated for a period of increasing magnetic activity. The prominent feature observed is the development of localized disturbance zones in the high-and mid-latitude regions which show a significant enhancement in argon and nitrogen densities and a simultaneous decrease in helium densities. The behavior of oxygen within these zones is complex, and both increases and decreases are observed. Both the latitudinal extent and the magnitude of the disturbed regions increase with growing magnetic activity. In contrast the low-latitude region exhibits moderate enhancements of all constituents. Using the AE index as an indicator of magnetic activity, we find that at higher latitudes the atmospheric response time is of the order of one orbital revolution or less. Comparisons with other observations and with a theoretical model by Mayr and Volland (1974) show good agreement.     

Transient thermospheric heating and movement caused by an auroral electric field

Two closed form solutions for the velocity distribution of the upper thermosphere were found using magnetohydrodynamic formalism. One corresponds to a constant altitude, is timedependent, and has non-moving boundaries. This case asympotically approaches the steady solution obtained by Cole (1971). The other solution corresponds to a time- and altitude-dependence case with free boundaries. Solutions of electrodynamic (joule) and viscous heating for both cases are given. Some numerical results corresponding to the latter case are presented. It is clearly demonstrated that joule heating is dominant within the electric field region, and that viscous heating becomes important in the neighbourhood of the electric field region. It is also shown that the induced movement extends beyond the electric field region as far as four times the original width of the electric field region.     

The response of the mid-latitude thermospheric wind to magnetic activity

Observations of the thermospheric wind at a mid-latitude station have been made using a Fabry-Perot interferometer to measure the Doppler shift of the nighttime OI emission at 630 nm. The results from 12 summer nights show that the zonal wind has a distinct feature associated with magnetic activity. The zonal wind first reverses and becomes westward. The maximum strength of the westward wind, its duration, and the maximum strength of the subsequent eastward wind all increase with increasing magnetic activity. The meridional wind is less consistent in its behaviour. It is normally equatorward but during magnetic activity it can increase, decrease, or even reverse, although it is consistently equatorward and of increased strength after 02.00 L.T. The initial reversal of the zonal wind is consistent with changes in the wind expected as a result of convective electric fields penetrating to mid-latitudes indicating that these electric fields modify the mid-latitude wind pattern before effects due to auroral heating reach mid-latitudes. The reversal of the zonal wind back to eastward may also be the result of electric field effects. The large variability of the meridional wind, to the extent that it becomes poleward at times, indicates the importance of wind sources equatorward of the observatory.     

Influence of gravity wave activity on lower thermospheric photochemistry and composition

The wind and temperature oscillations of internal gravity waves can cause horizontal variations of a factor of two in minor gas number densities in the lower thermosphere over length scales of several hundred kilometers. The variations are due both to vertical transport of constituents whose lifetimes are long compared to the wave period and to chemical activity driven by temperature dependent reaction rate coefficients. The nightglow emission of the hydroxyl radical provides a remote sensor of wave activity between 80 and 90 km. Theoretical calculations show that the horizontal variations in the atomic hydrogen distribution are the largest single contributor to wave structure in the nightglow followed by the effects of temperature fluctuations on the rate coefficient of the reaction H + O₃ → O₂ + OHv′ > 0).     

Decay of the magnetic storm ring current by the charge-exchange mechanism

Equatorial charge-exchange lifetimes of ring current protons are recalculated, and the decay of a collection of ring current protons trapped on an L-shell by the charge-exchange mechanism is determined using recent models of the hydrogen geocorona. Observational results pertaining to the decay of ring current energy are briefly discussed, as are a number of competing loss mechanisms. Since charge exchange is a simple physical process which is very efficient in removing ring current energy from L-shells near to the Earth (say, L < 4), it is suggested that it may well be the dominant loss mechanism in this region.     

Variations of the magnetic field near Mars caused by magnetic crustal anomalies

The possible avenues for photoelectron transport were determined during southern hemisphere winter at Mars by using a mapping analysis of the theoretical magnetic field. Magnetic field line tracing was performed by superposing two magnetic field models: (1) magnetic field derived from a three-dimensional (3D) self-consistent quasi-neutral hybrid model which does not contain the Martian crustal magnetic anomalies and (2) a 3D map of the magnetic field associated with the magnetic anomalies based on Mars Global Surveyor magnetic field measurements. It was found that magnetic field lines connected to the nightside of the planet are mainly channeled within the optical shadow of the magnetotail whereas magnetic field lines connected to the dayside of the planet are observed to form the remainder of the magnetosphere. The simulation suggests that the crustal anomalies create “a magnetic shield” by decreasing the region near Mars which is magnetically connected to the Martian magnetosphere. The rotation of Mars causes periodic changes in magnetic connectivity, but not to qualitative changes in the overall magnetic field draping around Mars.     

Line-of-sight magnetic flux imbalances caused by electric currents

Several physical and observational effects may contribute to the significant imbalances of magnetic flux that are often observed in active regions. We consider an effect not previously treated: the influence of electric currents in the photosphere. Electric currents can cause a line-of-sight flux imbalance because of the directionality of the magnetic field they produce. Currents associated with magnetic flux tubes produce larger imbalances than do smoothly-varying distributions of flux and current. We estimate the magnitude of this effect for current densities, total currents, and magnetic geometry consistent with observations. The expected imbalances lie approximately in the range 0–15%, depending on the character of the current-carrying fields and the angle from which they are viewed. Observationally, current-induced flux imbalances could be indicated by a statistical dependence of the imbalance on angular distance from disk center. A general study of magnetic flux balance in active regions is needed to determine the relative importance of other - probably larger -effects such as dilute flux (too weak to measure or rendered invisible by radiative transfer effects), merging with weak background fields, and long-range connections between active regions.Operated for the National Science Foundation by the Association of Universities for Research in Astronomy.     

Long periodic variations of magnetic stars caused by precession

Über lange Zeiten erstreckte Beobachtungen einiger Ap-Sterne ergeben deutliche säkulare Variationen der effektiven magnetischen Feldstärke, welche – neben anderen Interpretationsmöglichkeiten – auf die Präzessionsbewegung des Sterns zurückgeführt werden können. Die Präzession eines durch Rotation abgeflachten Sterns wird durch die Orbitalbewegung eines Begleiters mit einer gegen die Rotationsachese des Sterns geneigten Bahnebene oder durch Rotation der inneren gegenüber den äußeren Schichten des Sterns um unterschiedlich geneigte Achsen verursacht.     

Measurements of thermospheric temperatures at a mid-latitude station

Fabry-Perot interferometer measurements of the nightglow 630.0 nm line have been made at Beveridge(37°28′S, 145°6′E) from December 1980 to September 1981.Thermospheric temperatures have been derived from these measurements and compared to the MSIS model. Good agreement is found except during summer when the experimental temperatures are consistently higher (～ 100 K) than the model values. The experimental values are well described by a function similar to that used by Hernandez (1982b) to describe 7 years of data obtained at Fritz Peak which is at a similar mid-latitude to Beveridge. The fit to the Beveridge data indicates larger seasonal and magnetic variations in the temperature than given by the MSIS model.     

Multiperiodic irradiance changes caused by r-modes and g-modes

The acceleration of the influential 100 keV electrons in flares observed in hard X-rays and several radio emissions is unknown. Shock-waves and MHD turbulence, successfully applied to interprete interplanetary energetic particles, have recently been called in question concerning energetic flare electrons and ions. Other possible mechanisms are considered which are closely related to the primary flare energy release. In particular, runaway acceleration by the electric field of the reconnection current sheet, bulk heating by microturbulence, and cross-field ion currents due to bulk motion as a primary result of reconnection are reviewed. All three are likely to occur in some way. Their relative importance cannot be definitively assessed due to the lack of information on non-thermal, low energy protons.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Modelling of magnetic field measurements at Mercury

Mapping Mercury's internal magnetic field with a magnetometer in closed orbit around the planet will provide valuable information about its internal structure. By measuring magnetic field multipoles of order higher than the dipole we could, in principle, determine some properties, such as size and location, of the internal source. Here we try to quantify these expectations. Using conceptual models, we simulate the actual measurement during the BepiColombo mission, and then we analyze the simulated data in order to estimate the measurement errors due to the limited spatial sampling. We also investigate our ability to locate the field generating current system within the planet. Finally, we address the main limitation of our model, due to the presence of time-varying external magnetospheric currents.     

Diagnosis and simulation of a rapidly developing cyclone related to a severe dust storm in East Asia

Mongolian cyclones are important to the outbreaks of severe dust storms in Northeast Asia. In this paper, we conduct a diagnostic study and a numerical simulation of a rapidly developing Mongolian cyclone that produced the April 5 to 7, 2000 severe dust storm in East Asia. The surface pressure of the cyclone decreased rapidly on April 5 to 6 causing strong surface winds. The diagnosis shows that the cyclone was a “dry cyclone”, as the convergence of moisture flux was weak and the baroclinic forcing was strong. The analysis of the Q vectors also reveals that ageostrophic wind was significant in the middle and upper part of the cyclone, resulting in strong vertical motion in the lower part. The three-dimensional structure of the cyclone is characterized by an ascending southern warm current and a descending northern cold current. This structure is favorable to the release of available potential energy and the intensification of the cyclone. In the second part of this study, we develop a coupled system of a dust emission scheme and a mesoscale numerical model (MM5V3) and applied the system to the simulation of the Mongolian cyclone. The main features of the cyclone identified from the diagnostic analysis are successfully reproduced. The predicted dust storm-affected area is found to be consistent with the meteorological observations and satellite remote sensing. It is shown that the coupled system is capable of predicting the spatial and temporal variations of the dust storm.     

Possible changes of the AES orbit caused by a passage through the Moon's shadow

The change in the eccentricity of the AES orbit of the type 1963-30D as a result of one passage through the Moon's shadow is estimated, and it has been shown that the effect is not negligible.     

Shift in the bifurcation points of rotating magnetized newtonian polytropes caused by a magnetic field

Criteria are formulated for determining the critical points and bifurcation points of rotating, magnetized, newtonian polytropes, which coincide in the absence of a magnetic field. The magnitude of the shift in these points is estimated in terms of the flatness parameter e and rotation speed ε. The dependence of the total energy of a polytrope near the bifurcation and critical points is calculated as a function of the asymmetry parameters X for the distribution of mass relative to the axis of rotation and of the rotation speed ε for ε ≪ 1. The stability of the rotating polytrope with respect to the parameter X is analyzed. __________ Translated from Astrofizika, Vol. 51, No. 2, pp. 321–327 (May 2008).     

Insolation changes on pluto caused by orbital element variations

In this paper, we compare changes in the insolation at Pluto, corresponding to three epochs during the dynamical history of the planet: t = – 1, 0 and 0.5, where t is the time in millions of years A.D. The two extreme values of t coincide respectively with a maximum (126 ) and a minimum (102 ) value of the obliquity (). The other orbital elements i.e. the eccentricity (e) and the longitude of the perihelion ( _p ) which affect solar radiation and which are apt to significant periodic changes are also calculated for the times under consideration. In a series of figures, the combined influence of the evolving dynamic parameters on the daily insolation and on the mean (summer, winter, annual) daily insolation is illustrated.     

An example for solar flares caused by magnetic field non-equilibrium

Using a photospheric magnetogram of the solar activity complex HR 16862, 16863, 16864 at the end of May 1980 as boundary data a sequence of three-dimensional force-free magnetic fields with spatially constant ( defined by the equation × B = B) is calculated, including numerical field-line tracing. The field is assumed not to be frozen-in and ¦¦, which is a measure of the magnetic free energy, is assumed to increase with time due to some dynamo mechanism. Variation of , starting from = 0, produces a catastrophe-like change of the topology of a field-line system corresponding to an arch filament system connecting the leading spot of HR 16863 with the trailing spot of HR 16862. This topological change is interpreted as causing a series of large homologous flares with synchronous flaring in the two spots. The catastrophe-like behaviour of the field topology is attributed to the nonlinear field-line equations.     

Precipitation of auroral particles in the central polar cap during a sudden commencement magnetic storm

All-sky camera observations from two stations in the inner (northern) polar cap and an auroral zone station are combined with photometer records from the polar cap station Nord in a study of the brilliant auroral display following the ssc of the storm of 7 November 1970. This display is the large, poleward expanding bulge of a substorm triggered by the ssc. It is composed of brilliant discrete forms embedded in low-intensity diffuse electron and proton aurora. The poleward edge of the diffuse electron aurora is 5° north of the discrete auroras and 3° north of the proton aurora. The intensity of the discrete aurora varies as the strength of the auroral electrojet as shown by magnetograms from auroral zone stations. Succeeding the retreating display a subvisible low-energy electron precipitation, which may be identified as the polar squall (Winningham and Heikkila, 1974) is observed over the polar cap during the main phase of the storm.In the early morning sector already existing diffuse auroras broaden towards the equator from the time of the ssc and at least during the following half hour.Ssc-triggered displays have been found (Feldstein, 1959) to withdraw from the inner polar cap as the initial (positive H) phase of the storm ends. A comparison of the records from seven low-latitude stations shows that during this particular storm the positive phase appears to be composed by two overlapping disturbances, i.e. the proper initial phase, which is generally thought to be due to compression of the inner magnetosphere and a series of positive bays accompanying the negative bays in auroral latitudes. These positive bays are observable over a great range of longitudes with a maximum of amplitude near midnight. As judged from the dayside magnetograms the initial (compression) phase ends at an early stage of the substorm. The observed coincidence between the withdrawal of the display and the cessation of the positive H phase of the storm is a consequence of the fact that the second component—the positive bays—and the auroral display over the polar cap are both signatures of the substorm activity.     

Effect of field-aligned currents in the polar cap at the recovery phase of a magnetic storm

Unusually great fluctuations in the ΔB module of the geomagnetic field have been observed in the polar cap from the satellite Cosmos-321. They are explained by small-scale two-sheet field-aligned current systems which exist during the periods when magnetic fields having a considerable northward B_z(B_z 10 nT) component are observed in interplanetary space.