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1.
Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (Bz). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.  相似文献   

2.
Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H2 emission. The far ultraviolet color ratios I(1550-1620 Å)/I(1230-1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H2 spectra to fit the intensity distribution of the H2 lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm−2 s−1 flux of 80 keV electrons. It provides an effective H2 temperature of 540 K. The relative intensity of temperature-sensitive H2 lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH4, indicating a harder precipitation along high-latitude magnetic field lines.  相似文献   

3.
It is suggested that the quiet day daily magnetic variation in the polar cap region, Sqp, results partly from the short-circuit effect of the magnetotail current by the polar ionosphere. This implies that there is an inward field-aligned current from the dawnside magnetopause to the forenoon sector of the auroral oval (positively charged) and an outward field-aligned current to the duskside magnetopause from the afternoon sector of the oval (negatively charged), together with the ionospheric (Pedersen and Hall) currents. The distribution of the magnetic field vectors of both combined current systems agrees with the observed Sqpvector distribution. The space charges provide an electric field distribution which is similar to that which has been observed by polar orbiting satellites.  相似文献   

4.
The geometry of the open field line region in the polar region is computed for a variety of the interplanetary magnetic field (IMF) orientation. The open field line region can be identified as the area bounded by the auroral oval, namely the polar cap. The polar cap geometry varies considerably with the orientation of the IMF and magnitude, particularly when the IMF Bz component is positive and large. The corresponding exit points of the open field lines on the magnetopause are also examined. The results will be a useful guide in interpreting various upper atmospheric phenomena in the highest latitude region of the Earth and also in observing chemical releases outside the magnetopause.  相似文献   

5.
Assuming a certain horizontal distribution of the convection field at a certain altitude above the ionosphere, the associated electric field and current distributions in a vertical plane are calculated using a model with finite current-dependent conductivity along the magnetic field lines. It is seen that given the kind of horizontal distribution of E6 commonly observed by polar-orbiting satellites at inverted-V electron precipitation events, the calculated distribution of E∥ is able to reproduce the basic spatial structure of the precipitation. It is also seen that the combined effect of a locally increased ionization within auroral forms and a large potential difference (ΔV)∥ along the magnetic field lines at higher altitudes is a strong reduction of E6 within the auroral forms. From the basic features of the electric field, it is concluded that an interpretation of auroral precipitation in terms of a static E∥ may require a mechanism that can support a large (ΔV)∥ even at relatively weak current densities and at the same time allow local enhancements of the parallel conductivity within the region of non-zero E∥. It is suggested that the magnetic mirroring combined with gyro-resonant wave-particle interactions may be a suitable mechanism.  相似文献   

6.
An extensive study of DMSP photographs and the simultaneous interplanetary magnetic field data suggests that the quantity defined by
S=∫τ0(ФD ? ФN)dt
has a fundamental importance in substorm processes, where ΦD and ΦN denote the production rate of merged (or open) field lines along the dayside X-line and of reconnected (or closed) field lines along the nightside X-line, respectively; t = 0 is measured from the time when the Bz component begins to decrease after a prolonged period of a large positive Bz value. It is shown, first of all, that substorms occur so long as S > 0, regardless of the sign of the Bz component and its changes (namely, the southward and northward turnings) and of its time derivative as well. Secondly, the intensity of substorms is proportional to S2. By introducing the quantity S, the recent confusion of the problem of the roles of the north-south component of the interplanetary magnetic field on substorm processes can be removed.Since S is equal to the amount of the open magnetic fluxes at a time reckoned from t = 0, it is proportional to (A1 ? A0), where A0 denotes the minimum polar cap area (namely, the area bounded by the minimum auroral oval) and A1 the polar cap area at an arbitrary time t. Therefore, substorms can occur whenever the auroral oval is larger than its minimum size. Further, an intense substorm tends to occur along a large oval.The quantity S can also be considered as an excess flux, and thus the substorm can be considered as a process by which the magnetosphere tends to remove sporadically the excess energy associated with S.  相似文献   

7.
On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (Sqp) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of Sqp is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.  相似文献   

8.
Images of the instantaneous nightside auroral distribution reveal that at times the orientation of auroral oval arcs changes to become characteristic of polar cap arcs. These connecting arcs all terminate in the diffuse aurora in the midnight sector, and their separation from the equatorward boundary of the diffuse aurora generally increases away from the midnight termination. The occurrence of these features requires a northward interplanetary magnetic field (positive Bz) as well as low magnetic activity. The existence of connecting arcs and the observation that they are at times the poleward boundary of weak diffuse emission indicate that the poleward boundary of auroral emissions can be significantly modified during non-substorm periods. Such a distortion implies that there can be a modification of the standard convection pattern in the magnetosphere during periods of positive Bz to produce expanded regions of sunward convection in the high latitude ionosphere.  相似文献   

9.
Two kinetic models for the auroral topside ionosphere are compared. The collisionless plasma distributed along an auroral magnetic field line behaves like a non-Ohmic conducting medium with highly non-linear characteristic curves relating the parallel current density to the potential difference between the cold ionosphere and the hot plasmasheet region. The (zero-electric current) potential difference, required to balance the current carried by the precipitating plasmasheet particles and the current transported by the outflowing ionospheric particles, depends on the ratio nps.e/nth.e and Tps.e/Tth.e of the plasmasheet and ionospheric electron densities and temperatures. When in the E-region the magnetic field lines are interconnected by a high conductivity plasma the resulting field-aligned currents driven by the magnetospheric potential distribution are limited by the integrated Pedersen conductivity of the ionospheric layers. These currents are not related to the parallel electric field intensity as they would be in Ohmic materials. The parallel electric field intensity is necessarily determined by the local quasi-neutrality of the plasma.  相似文献   

10.
The distribution of large-scale field-aligned currents in the dayside sector of the auroral oval has been presented for different situations in the interplanetary space. The j∥ distribution has been calculated on the basis of a model, each part of which is controlled by a corresponding parameter of the interplanetary space. It has been shown that the field-aligned current models, proposed by Iijima and Potemra and by McDiarmid et al. describe the planetary j∥ distribution for only particular situations in the interplanetary space and represent some particular cases of a more general model.  相似文献   

11.
This paper reports the study concerning the latitudinal dispalacement of the auroral oval as a function of the northward orientation of the Bz-component IMF and the relation between southward Bz and the auroral dynamics in the night sector.  相似文献   

12.
One of the most striking and persistent features in high latitude regions as seen by the ISIS-2 scanning auroral photometer is a fairly uniform belt of diffuse auroral emission extending along the auroral oval. Indications are that this region follows, contributes to, and may in a sense actually define the auroral oval during quiet times.The diffuse belt is sharply defined at its equatorward edge, which is located at an invariant latitude of about 65° in the midnight sector during relatively low magnetic activity (Kp = 1?3). The poleward edge of the region is not as sharply defined but is typically at about 68°. Discrete auroras (arcs and bands) are located, in general, near the poleward boundary of the diffuse aurora. The position of the belt appears to be relatively unaffected by the occurrence of individual substorms, even when discrete forms have moved well poleward. Representative intensities at 5577 Å are 1–2 kR (corrected for albedo) at quiet times and may reach 5 kR during an auroral substorm.It appears that the mantle aurora and proton aurora constitute this diffuse aurora in the midnight sector. Precipitating protons and electrons both contribute to the emissions in this region.  相似文献   

13.
We describe a simple method for determining the time at which the meridian of a sub-auroral magnetic observatory crosses that of the Harang discontinuity—the separation of the eastward and westward electrojets which flow in the evening and morning sectors of the auroral oval. We then consider how this time, determined from examination of magnetograms from sub-auroral observatories varies with the dawn-dusk (y) component of the Interplanetary Magnetic Field. We find that the time at which the Harang discontinuity is identified in the Northern Hemisphere is earlier for By > 0 than the occasions when By < 0, and that the converse is observed in the Southern Hemisphere. Also we suggest that there is no significant seasonal variation in the relationship between the time of the discontinuity and By. The sense of the azimuthal shift of the auroral electrojet currents with changes in By is consistent with the theory of Cowley (1981). However, the magnitude of the observed shifts is approximately an order of magnitude greater than the theoretical predictions. We suggest that this difference between observation and theory arises from the use of a dipole magnetic field model at auroral zone latitudes in the theoretical estimation of azimuthal displacement.  相似文献   

14.
《Planetary and Space Science》1987,35(8):1009-1020
Latitudinal structures of discrete arcs are modelled as a consequence of the quasi-steady magnetosphere-ionosphere coupling involving viscous interaction between sunward and anti-sunward plasma flows in the magnetosphere. The quasi-steady state in the magnetosphere and ionosphere coupling is described by the magnetospheric and ionospheric current conservation and the field-aligned currentpotential relation assuming adiabatic electron motion along field lines. The upward and downward fieldaligned currents are assumed to be stably maintained by vorticity-induced space charges in the region of plasma flow reversal, where divergence of the magnetospheric electric field E⊥ is negative and positive, respectively. By introducing the effective conductance Σdc arising from the anomalous viscosity, a specific relation between the dc field-aligned current density J∥ and the magnetospheric electric field E⊥ is derived as J∥=−ΣdcdivE⊥. Sufficiently large potential drops to accelerate auroral electrons are shown to exist along the auroral field lines originating from the flow reversal region with div E⊥ < 0. It is shown that the latitudinal structure of a discrete arc is primarily determined by the magnetospheric potential structure and the characteristic width is on the order of 10 km at the ionospheric altitude.  相似文献   

15.
Global auroral imagery obtained by DMSP satellites during the years 1972–1979 over both the northern and southern high latitude polar regions were examined to study the morphology of the discrete arcs known as polar cap arcs. Based upon their morphology, the polar cap arcs can be generally classified into three types viz. (1) the distinctly sun-aligned polar cap arcs—Type 1 arcs, (2) the morning/evening polar cap arcs expanded from the auroral oval—Type 2 arcs and (3) the hook shaped arcs connecting the polar cap arc with the oval arc (including the hitherto unreported oppositely oriented hook shaped arcs)—Type 3 arcs. Concurrent auroral electrojet indices (AE) and interplanetary magnetic field (IMF) data were used to study the occurrence of the polar cap arcs. It was found that Type 1 arcs were observed mostly during low geomagnetic activity conditions, bright Type 2 arcs during the recovery phase of the substorms and Type 3 arcs do not occur during the recovery phase of the substorm. Over both hemispheres, the polar cap arcs were observed mostly during northward IMF. Furthermore, Type 1 arcs were obeserved over the northern polar cap during mostly negative Bx periods and over the southern polar cap during mostly positive Bx periods. The latter observation suggests that these types of arcs may be non-conjugate.  相似文献   

16.
The direction of motion of the auroral forms in several sectors of the auroral oval during substorms is studied. The creation phase is characterized by the equatorward displacement of the luminous region in evening (15–21 LT) and in day (09–15 LT) hours, while individual forms in the luminous region drift mainly poleward with a mean velocity of 230 m/sec in day hours and equatorward with the mean velocity of 230 m/sec in evening hours. The equatorial shift of the luminous region correlates well with the BZ-component of the interplanetary magnetic field. The onset of the displacement coincides with the southward BZ-rotation and is accompanied by auroral intensity increase for about 10–20 min.During the expansive and recovery phases the day auroras drift poleward with mean velocities of 330 and 300 m/sec, respectively. In the evening sector the individual auroral forms drift both poleward and equatorward during the expansive phase and drift mainly towards the pole during the recovery phase with a mean velocity of 200 m/sec. In the morning sector characteristics of the motion of the individual auroral forms are more complicated than in the other sectors. The well defined shifts of the luminous region are not discovered. The possible relation between the motions of individual auroral forms with the magnetosphere convection is discussed.  相似文献   

17.
We present a detailed analysis of the H+3 intensity and velocity profiles crossing Saturn's auroral/polar region, as described by Stallard et al. [Stallard, T., Miller, S., Melin, H., Lystrup, M., Dougherty, M., Achilleos, N., 2007. Icarus 189, 1-13], with a view to understanding the magnetospheric processes with which they are connected. The data are not consistent with the theory that Saturn's main auroral oval is associated with corotation enforcement currents in the middle magnetosphere. This implies that the main auroral oval can be associated with the open-closed field line boundary [Cowley, S.W.H., Bunce, E.J., O'Rourke, J.M., 2004. J. Geophys. Res. 109. A05212]; a third model, by Sittler et al. [Sittler, E.C., Blanc, M.F., Richardson, J.D., 2006. J. Geophys. Res. 111. A06208] associates the main oval with centrifugal instabilities in the outer magnetosphere, but does not make predictions about ionospheric plasma flows with which we can compare our data. We do, however, tentatively identify emission at latitudes lower than the main auroral oval which may be associated with the corotation enforcement currents in the middle magnetosphere. We also find that at latitudes higher than the main auroral oval there is often a region of the ionosphere that is in rigid corotation with the planet. We suggest that this region corresponds to field lines embedded in the centre of the magnetotail which are shielded from the solar wind such that their rotation is controlled only by the neutral atmosphere.  相似文献   

18.
Computerized spherical harmonic analysis is applied to the morphology of the southern auroral oval. Records from 23 All Sky Camera stations together with visual observation reports for the period 1957–1959 constitute the raw data set. The mode of the derived auroral occurrence distribution function F(Kp, θ, φ) is regarded as the maximum probability contour and yields a set of auroral ovals. These 10 contours, one for each Kp level, are expressed in the invariant magnetic co-ordinates of Bond (1968).  相似文献   

19.
Equations governing the large-scale electrodynamic processes in the auroral ionosphere are systematically discussed and the limits to drawing conclusions from incomplete sets of equations are evaluated. The vectors of electic current density,j, and electric field,E, are expressed as explicit functions of the densities, pressures and velocities of the constituents of the ionosphere.The equation div (·E)=0 is an identity satisfied by any solution of the full set of equations governing the problem and cannot be treated as a differential equation forE in which the components of the conductivity tensor are given parameters. The concept of the height-integrated conductivities and the conclusions based on it are inconsistent with the equations of momentum balance for the ionospheric constituents.The global structure of the auroral ionosphere is determined by the state of equilibrium between the pressure gradients, the inertial forces and thej×B-force associated with the auronal electrojets flowing along the auroral oval. The time-averaged, global, electric field is directed across the auroral oval. Its value is substantially affected by the motions of neutral particles. The velocity vector of the neutrals has a substantial component directed across the oval.  相似文献   

20.
In view of observations which show that a substorm often begins in a small local time sector, a model is assumed in which the neutral sheet current is diverted around a small region we call a bubble. The simplest assumption is that of a linear variation of current with distance from the centre of the bubble in the x-direction in a SM coordinate system, with the diverted current being channelled within narrow paths of width δy on the dawn and dusk sides of the bubble. This assumption leads to vector potential integrals that can be evaluated analytically. The addition of this current loop into the magnetotail results in a magnetic field structure where new neutral lines of X- and 0-type can be observed; these are connected to each other as a continuous neutral ring in the xy equatorial plane. The magnetic and electric field components around the neutral regions are calculated, and the time dependent evolution of the neutral ring is studied. Comparison with some published satellite observations shows good agreement. Taking typical values for the various quantities on the basis of actual observations within the magnetotail, we show that the induced electric field is at least comparable to the average cross-tail electrostatic field, and it may well be one or two orders of magnitude greater. The response of the plasma to the induction field is discussed qualitatively. It is concluded that field aligned currents may be produced due to inertial forces of the expanding disturbance. Interpretation of the ground based precipitation patterns of energized particles during auroral breakup is given.  相似文献   

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