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1.
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
2.
P.E. Sandholt A. Egeland B. Lybekk C.S. Deehr G.G. Sivjee G.J. Romick 《Planetary and Space Science》1983,31(11):1345-1362
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
3.
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
4.
A study of ground-based all-sky photographs substantiates the presence of the diffuse auroral belt as seen by the ISIS-2 (polar orbiting satellite) scanning auroral photometer. The intensity of the diffuse aurora increases when discrete auroras become active; in particular the diffuse aurora is most clearly seen equatorward of westward travelling surges. However, in the morning sector, it may or may not be detectable near eastward drifting patches in all-sky photographs. Some of what has been previously identified visually and in all-sky photographs as the proton aurora probably is a part of what we identify here as the diffuse aurora.The diffuse aurora appears sometimes to branch out into two, one along the auroral oval and the other along a constant geomagnetic latitude circle. The latter probably corresponds to the mantle aurora and the drizzle zone precipitation. 相似文献
5.
The behaviour of the polar auroras in the dark part of the auroral oval during the solar flares has been examined. For the analysis 29 solar flares during spring and autumn periods when a part of the polar cap was sunlit were selected. It has been found that a sharp decreasing of the auroral arc luminosity occurred just after the solar flare onsets. Auroral arcs broke up into patches and in most cases disappeared in 2–3 min. Bright discrete auroras appeared again as a rule close to the maximum phase of the solar flares. The duration of polar aurora effects was typically from 4 to 13 min with median value of about 8 min. These effects have been observed inside the interval 18.00-04.00 M.L.T. during periods both of magnetic quiet and disturbance.For the large set of data magnetic field variations in the sunlit polar cap after the solar flare onset have been investigated. A simple model of the auroral processes for the qualitative explanation of the observed phenomenon has been suggested. 相似文献
6.
Ionogram and all-sky camera data have been recorded on the Air Force Cambridge Research Laboratories' Flying Ionospheric Laboratory in the day sector of the auroral oval under conditions of darkness. The airborne measurements show that the polar F-layer irregularity zone, which is characterized on ionograms by a generally non-retarded and spread F type echo, exhibits meridional motions similar to the day-sector auroras. The polar F-layer irregularity zone and the day-sector auroras move equatorward and then move poleward in harmony with the development and decay of a magnetospheric substorm. We suggest that the polar cusp also moves in essentially the same fashion. 相似文献
7.
Knowledge of the structure of the polar ionosphere during exceptionally quiet periods is basic for studying complicated ionospheric behaviors during disturbances. On the basis of data from an airborne ionosonde as well as a meridian chain of ground-basedionosondes, the circumpolar structure of the E,-and F-regions is elucidated. There are two circumpolar zones of E-region ionization with differing characteristics. The first is an auroral E,-layer and/or retarded type sporadic E-band that has previously (Whalen et al., 1971) been found to be identical with the continuous aurora. The second is a zone of non-retarded type spora die E located poleward of the former band. In general, discrete auroras are co-located with the latter. The main trough, a prominent feature of the night sector F-region, is most pronounced in the early morning. The main trough is bounded on the poleward side by a well defined ‘wall’ of F-region ionization. The night sector poleward trough wall is located approximately three degrees of latitude equatorward of the auroral oval. A ‘plateau’ of F-region ionization extends from the poleward trough wall to the auroral oval. 相似文献
8.
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
9.
William N. Hall 《Planetary and Space Science》1974,22(9):1315-1321
Pulsating auroras were recorded at Bedford, Massachusetts, cgm lat. 55.4°, 24 March 1969 during a worldwide magnetic storm, the only known published observations of pulsating auroras at such low magnetic latitudes. Spectral density analysis of several minutes of 5577A pulsations indicated a dominant period of 7.2 sec at 0300 EST. The following characteristics were noted: (1) occurrence during a negative bay in H; (2) location toward the equatorial boundary of the auroral display; (3) occurred a few hours after local midnight; (4) characteristic period of 6–10 sec; (5) quasi-sinusoidal or superposition of sinusoids rather than isolated pulses; (6) modulation of the background intensity by 15–30 per cent. These characteristics have previously been observed by others in pulsating auroras in the auroral oval. Other mid-latitude geophysical measurements at the same time show similarities to typical auroral oval behavior. These observations indicate that the auroral oval expanded during the worldwide magnetic storm until the boundary of the auroral oval was near cgm latitude 55°. If this observation of one mid-latitude pulsating aurora is in general valid, then the agreement of the characteristic period of pulsating auroras when the oval has expanded to mid-latitudes with the period of pulsating auroras when the oval is not expanded should be useful in distinguishing between proposed source mechanisms for these pulsating auroras. 相似文献
10.
The ISIS-2 scanning auroral photometer surveyed the polar region during three successive passes on 18 December 1971, at times when Kp values were still high due to an intense magnetic storm which began on 16 December. Two very bright (IBC III) auroral substorm patterns were seen to correspond to rather weak magnetic substorms (about 300 γ in magnitude). A large spiral auroral pattern, with intensity of the order of 100 kR and a size of about 1300 km, was present in the polar cap; it gradually decreased in size and intensity during the interval 0200–0600 UT. A region of enhanced 3914 emission was present in the noon sector of the auroral oval between 0200 and 0400. The presence of the diffuse auroral belt is also evident at all local times during this period, extending down to about 61° corrected geomagnetic latitude in the midnight sector. 相似文献
11.
The poleward boundary of the auroral oval, whose footline forms the periphery of the polar cap, is calculated, based on a model in which the geomagnetic field is interpermeated with the interplanetary field. It is shown that the calculated auroral oval size varies with the strength and direction of the interplanetary magnetic field, in agreement with recent observations of the location of large-scale nightside auroras. 相似文献
12.
Synoptic auroral photographs acquired by a United States Air Force DMSP satellite have provided the data necessary for an improved understanding of the common auroral phenomena that occur during the substorm expansive phase. Specifically, it is recognized that diffuse auroras (patches, large scale wave structures and Omega bands) form an integral part of the auroral substorm and that their morphological features should be added in the substorm pattern proposed by Akasofu (1964). Several other important auroral substorm features have been added to or corrected in the original pattern. A revised substorm pattern at about the maximum epoch of the substorm is presented. 相似文献
13.
The ground signatures of multiple onset substorms have been investigated in night-side magnetograms from low to high latitudes and in observations of auroral-zone electron precipitation. Pi 2 onsets at three widely spaced stations are used for accurate timing of each onset. It is found that an evening auroral arc brightens at the onset of each Pi 2 train, also in the case of weak pulsations before the first low-latitude positive bay onset. The latter onset is, on the other hand, associated with the initiation of a westward travelling surge, and field-aligned currents moving with the surge cause a similar westward movement of the magnetic signatures in subauroral and low-latitude magnetograms. At the arrival of a surge at an evening side observatory, the westward electrojet is displaced rapidly poleward, with a sharp increase in local bay activity and high-energy electron precipitation. The westward expansion of new activity appears as a continuous motion along the oval and is associated with a local poleward displacement of the westward electrojet. Consecutive surge initiation and low-latitude onsets do not, however, always occur progressively farther west. Thus, the development of the expansion phase consists of a series of intensifications and auroral surge formations at 10–20 min intervals. Near the time of maximum auroral-zone bay activity and apparently also when maximum westward extent is reached, the whole nighttime oval seems to be shifted poleward. Our findings are thus not consistent with the Wiens and Rostoker (1975) northward-westward stepping model. An alternative model is therefore presented based on the fundamental role of the westward travelling surge in carrying substorm activity westward along the oval. The associated field-aligned current system will perturb the pre-existing magnetospheric current wedge and cause positive bay increases at low latitudes and westward moving magnetic signatures at subauroral stations. 相似文献
14.
On the basis of the geomagnetic data of highlatitude arctic stations the development of polar magnetic substorms is examined. It is shown that there exist two current systems of the magnetic substorm: DP11 and DP12. 11 is a current system with one westelectrojet in the nighttime auroral zone. That system is peculiar to the break-up phase of a substorm. DP12 is a two-vortex current system in the polar cap with two auroral electrojets, eastward and westward, of about equal intensity. The DP12 system is typical for growth and recovery phases.There are two different types of substorm development. The first type is characterized by the DP12 system during the growth phase. The intensity of this current system increases until the explosive phase begins. The other type does not seem to be characterized by any distinct current system during the growth phase. The commencement of such a substorm is associated with a rapid explosive development of the DP12 system.A conclusion about the the different origins of the DP11 and DP12 current systems is made. 相似文献
15.
This paper presents a brief summary of an extensive correlative study of ATS-5 particle and magnetic field data with all-sky photographs from Great Whale River which is near the ‘foot’ of the field lines passing through the ATS-5 satellite. In particular, an effort is made to identify specific particle features with specific auroral displays during substorms, such as a westward travelling surge, poleward expansive motion and drifting patches. Some of the important findings are (i) in early evening hours, the first encounter of ATS-5 with hot plasma is associated with the equatorward shift of the diffuse aurora, but not necessarily with westward travelling surges (even when the satellite is embedded in the plasma sheet.) (ii) In the midnight sector, an injection corresponds very well to the initial brightening of an auroral arc. (iii) Specific features of morning sector auroras (for example, drifting patches) are difficult to correlate with specific particle features (gross features, but not specific).Comparing these results with particle data from low-latitude polar orbiting satellites, it is concluded that the plasma sheet near the earthward edge (consisting of plasmas injected during earlier substorms) is little affected during substorms. 相似文献
16.
We propose a model three-dimensional current system for the magnetospheric substorm, which can account for the new findings of the field-aligned and ionospheric currents obtained during the last few years by using new techniques. They include (1) the ionospheric currents at the auroral latitude deduced from the Chatanika incoherent scatter radar data, (2) the field-aligned currents inferred from the vector magnetic field observations by the TRIAD satellite and (3) the global distribution of auroras with respect to the auroral electrojets appearing in DMSP satellite photographs. The model current system is also tested by a computer model calculation of the ionospheric current pattern. It is shown that the auroral electrojets have a strong asymmetry with respect to the midnight meridian. The westward electrojet flows along the discrete aurora in the evening sector, as well as along the diffuse aurora in the morning sector. The eastward electrojet flows equatorward of the westward electrojet in the evening sector. It has a northward component and joins the westward electrojet by turning westward across the Harang discontinuity. Thus, the latitudinal width of the westward electrojet in the morning sector is much larger than that in the evening sector. The field-aligned currents, consisting of two pairs of upward and inward currents (one is located in the morning sector and the other in the evening sector), are closed neither simply by the east-west ionospheric currents nor by the north-south currents, but by a complicated combination of the north-south and east-west paths in the ionosphere. The magnetospheric extension of the current system is also briefly discussed. 相似文献
17.
B. Thiele R. BostrÖM A. Dumbs K.U. Grossmann D. Krankowsky P. LÄmmerzahl G. Marklund E. Neske G. Schmidtke K. Wilhelm 《Planetary and Space Science》1981,29(4):455-468
Four sounding rocket payloads were launched in early 1977 to measure heating parameters in the auroral oval. Geophysical conditions were different for the four flights: auroral arc substorm main phase diffuse aurora, and auroral arc with negative bay. The conductivity tensor and the heating rates of particle and Joule heating are determined. The heating rates range in the order of a few tens of mWm?2. These magnitudes accord with those determined with the aid of backscatter facilities and other sounding rocket observations. 相似文献
18.
The location of field-aligned currents in the evening sector with respect to the auroral electrojets is examined. The tri-axial TRIAD satellite data and the simultaneous ground magnetometer data from along the Alaska meridian are analysed. It is shown that an intense upward fieldaligned current flows out from the region of the westward electrojet where discrete auroras are located. The downward flowing current exists in the region further equatorward, namely in the region of the eastward electrojet. However, the downward current is present even when there is no eastward electrojet. The boundary between the upward and the downward currents coincides, in most cases, with the boundary between the westward and the eastward auroral electrojets. Thus, the Harang discontinuity, a narrow area separating the positive and negative H bays, is the region where there is no field-aligned current. 相似文献
19.
A.M. Du W. Sun B.T. Tsurutani R.N. Boroyev A.V. Moiseyev 《Planetary and Space Science》2011,59(13):1551-1558
A new type of polar cap aurora, dawn–dusk aligned polar cap aurora (DDAPCA), was detected during the exceptionally intense January 21, 2005 substorm (AEmax=3504 nT). The DDAPCA was located at very high latitude (>85° MLAT) in the polar cap region. As the interplanetary magnetic field (IMF) GSM By component rotated from a positive to a negative value, the DDAPCA tilt angle relative to the dawn–dusk direction rotated anticlockwise and reached ∼45°. It is speculated that the DDAPCA arises from the formation of an X-line in the distant (>80RE) tail due to polar cap magnetic field reconnection under unusually high solar wind compression conditions. 相似文献
20.
G. Rostoker A.J. Chen F. Yasuhara S.-I. Akasofu K. Kawasaki 《Planetary and Space Science》1974,22(3):427-437
The magnetic perturbation patterns in the polar cap and auroral zone regions are obtained for extremely quiet days using two different techniques. It is shown that the form of the equivalent current flow pattern is extremely sensitive to the level of quietness, and that even so-called quiet days are at times disturbed by substorm activity. Certain characteristic equivalent flow not typically observed during substorms is noted in the polar cap, and this flow appears to be associated with effects associated with polar cap perturbations discussed by Svalgaard (1973). As well a region of equatorward flow appears at high latitudes near the dawn meridian, which appears to be Hall current driven by an eastward electric field. The dayside sub-auroral zone is dominated by the Sq-current system, while the nightside shows no significant current flow in the absence of substorm activity. 相似文献