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1.
The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath ones, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent middle- to high-altitude observations (i.e., the Interball, Hawkeye, Polar, Image, and Cluster spacecraft) reported the cusps to encompass a broad area near local noon. The present paper focuses on a statistical study of the high-altitude cusp and surrounding magnetosheath regions as well as on some peculiarities of the cusp-magnetosheath transition. For a comparison of high- and low-altitude cusp determination, we present a mapping of two-year Magion-4 (a part of the Interball project) observations of cusp-like plasma along model magnetic field lines (according to the Tsyganenko 96 model) down to the Earth’s surface. The footprint positions show a substantial latitudinal dependence on the dipole tilt angle. The dependence can be fitted by a line with a slope of 0.14° MLAT per 1° of tilt. In contrary to previously reported IMF or solar wind influences on the cusp shape or location, some differences exist: (1) a possible IMF BX dependence of the cusp location, (2) a split cusp for BY≠ 0, and (3) a smaller cusp during periods of higher solar wind dynamic pressure. The conclusions following from the statistical analysis are confirmed by case studies which reveal the physical mechanisms leading to the observed phenomena. Results have shown that (1) reconnection near the cusp does not necessarily lead to observable precipitation, (2) the cusp precipitation in one hemisphere can be supplied from the conjugate hemisphere, and (3) the cusp geometry at a certain time depends on the IMF history.  相似文献   

2.
Energetic electrons (e.g., 50 keV) travel along field lines with a high speed of around 20 REs−1. These swift electrons trace out field lines in the magnetosphere in a rather short time, and therefore can provide nearly instantaneous information about the changes in the field configuration in regions of geospace. The energetic electrons in the high latitude boundary regions (including the cusp) have been examined in detail by using Cluster/RAPID data for four consecutive high latitude/cusp crossings between 16 March and 19 March 2001. Energetic electrons with high and stable fluxes were observed in the time interval when the IMF had a predominately positive Bz component. These electrons appeared to be associated with a lower plasma density exhibiting no obvious tailward plasma flow (<20 keV). On the other hand, no electrons or only spike-like electron events have been observed in the cusp region during southward IMF. At that time, the plasma density was as high as that in the magnetosheath and was associated with a clear tailward flow. The fact that no stable energetic electron fluxes were observed during southward IMF indicates that the cusp has an open field line geometry. The observations indicate that both the South and North high latitude magnetospheric boundary regions (including both North and South cusp) can be energetic particle trapping regions. The energetic electron observations provide new ways to investigate the dynamic cusp processes. Finally, trajectory tracing of test particles has been performed using the Tsyganenko 96 model; this demonstrates that energetic particles (both ions and electrons) may be indeed trapped in the high latitude magnetosphere.  相似文献   

3.
Observations of a unique cusp feature at low and mid altitudes are reported. This feature has a consistent double-peaked or “V”-shaped structure at the equatorward edge of high-latitude particle precipitation flux, and is predominantly present for high IMF By conditions. The observations are consistent with the Crooker (‘A split separator line merging model of the dayside magnetopause’, J. Geophys. Res. 90 (1985) 12104, ‘Mapping the merging potential from the magnetopause to the ionosphere through the dayside cusp’, J. Geophys. Res. (1988) 93 7338.) antiparallel merging model, which predicts a narrow wedge-shaped cusp whose geometry depends greatly on the dawn/dusk component of the IMF. Various observations are presented at low altitudes (DE-2, Astrid-2, Munin, UARS, DMSP) and at mid altitudes (DE-1, Cluster) that suggest a highly coherent cusp feature that is consistent with the narrow, wedge-shaped cusp of Crooker (1988), and contains persistent wave signatures that are compatible with previously reported high-altitude measurements. A statistical survey of Astrid-2 and DMSP satellite data is also presented, which shows this feature to be persistent and dependent on the IMF angle at the magnetopause, as expected. Thus, the cusp signatures observed at a wide range of altitudes present a coherent picture that may be interpreted in terms of a footprint of the magnetopause current layer.  相似文献   

4.
We report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Troms and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Ålesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened field lines for the observed negative IMF By. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating, northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward Bz, and explain their morphology in the context of previous theoretical work.  相似文献   

5.
Cusp geometry in MHD simulations   总被引：2，自引：0，他引：2  
The MHD simulations described here show that the latitude of the high-altitude cusp decreases as the IMF swings from North to South, that there is a pronounced dawn–dusk asymmetry at high-altitude associated with a dawn–dusk component of the IMF, and that at the same time there is also a pronounced dawn–dusk asymmetry at low-altitude. The simulations generate a feature that represents what has been called the cleft. It appears as a tail (when the IMF has a By component) attached to the cusp, extending either toward the dawn flank or the dusk flank depending on the dawn–dusk orientation of the IMF. This one-sided cleft connects the cusp to the magnetospheric sash. We compare cusp geometry predicted by MHD simulations against published observations based on Hawkeye and DMSP data. Regarding the high-altitude predictions, the comparisons are not definitive, mainly because the observations are incomplete or mutually inconsistent. Regarding the low-altitude prediction of a strong dawn–dusk asymmetry, the observations are unambiguous and are in good qualitative agreement with the prediction.  相似文献   

6.
We investigate the dayside auroral dynamics and ionospheric convection during an interval when the interplanetary magnetic field (IMF) had predominantly a positive Bz component (northward IMF) but varying By. Polar UVI observations of the Northern Hemisphere auroral emission indicate the existence of a region of luminosity near local noon at latitudes poleward of the dayside auroral oval, which we interpret as the ionospheric footprint of a high-latitude reconnection site. The large field-of-view afforded by the satellite-borne imager allows an unprecedented determination of the dynamics of this region, which has not previously been possible with ground-based observations. The location of the emission in latitude and magnetic local time varies in response to changes in the orientation of the IMF; the cusp MLT and the IMF By component are especially well correlated, the emission being located in the pre- or post-noon sectors for By < 0 nT or By > 0 nT, respectively. Simultaneous ground-based observations of the ionospheric plasma drift are provided by the CUTLASS Finland HF coherent radar. For an interval of IMF By 0 nT, these convection flow measurements suggest the presence of a clockwise-rotating lobe cell contained within the pre-noon dayside polar cap, with a flow reversal closely co-located with the high-latitude luminosity region. This pattern is largely consistent with recent theoretical predictions of the convection flow during northward IMF. We believe that this represents the first direct measurement of the convection flow at the imaged location of the footprint of the high-latitude reconnection site.  相似文献   

7.
The effect of the interplanetary parameters on the latitudinal position of the substorm westward electrojet is studied in the work. The data from the IMAGE chain of magnetic stations and POLAR and WIND satellites for the period close to the solar activity minimum (1995–1996) and for the period of the solar activity maximum (2000) have been used for this purpose. It has been indicated that the electrojet poleward edge reaches, on average, higher latitudes at a higher solar wind velocity and at a larger (B s ) IMF southward component. It has been indicated that the average latitude of the westward electrojet center increases with increasing solar wind velocity and decreases with increasing IMF southward component, as a result of which the electrojet center is, specifically, not observed at high geomagnetic latitudes at large values of the IMF southward component.  相似文献   

8.
The STARE system (Scandinavian Twin Auroral Radar Experiment) provides estimates of electron drift velocities, and hence also of the electric field in the high-latitude E-region ionosphere between 65 and 70 degrees latitude. The occurrence of drift velocities larger than about 400 m/s (equivalent to an electric field of 20 mV/m) have been correlated with the magnitude of the Interplanetary Magnetic Field (IMF) components Bz and By at all local times. Observation days have been considered during which both southward (Bz<0) and northward (Bz>0) IMF occurred. The occurrence of electric fields larger than 20 mV/m increases with increases in Bz magnitudes when Bz<0. It is found that the effects of southward IMF continue for some time following the northward turnings of the IMF. In order to eliminate such residual effects for Bz<0, we have, in the second part of the study, considered those days which were characterized by a pure northward IMF. The occurrence is considerably lower during times when Bz>0, than during those when Bz is negative. These results are related to the expansion and contraction of the auroral oval. The different percentage occurrences of large electric field for By>0 and By<0 components of the IMF during times when Bz>0, clearly display a dawn-dusk asymmetry of plasma flow in the ionosphere. The effects of the time-varying solar-wind speed, density, IMF fluctuations, and magnetospheric substorms on the occurrence of auroral-backscatter observations are also discussed.  相似文献   

9.
HF radar data during equinoctial, small IMF By conditions have enabled the ionospheric convection during the substorm growth phase and substorm pseudobreakup to be studied in both hemispheres. This has revealed both conjugate and non-conjugate convection behaviour during the substorm growth phase before and after the pseudobreakup onset. The nightside convection pattern is found to respond promptly to the southward turning of the interplanetary magnetic field (IMF) which impacts on the dusk flank of the magnetosphere due to an inclined phase front in the IMF in the case study presented. The subsequent interhemispheric observations of nightside convection are controlled by the IMF By polarity. The time scale for the response to changes in the IMF By component is found to be a little longer than for Bz, and the full impact of the IMF By is not apparent in the nightside convection until after substorm pseudobreakup has occurred. The pseudobreakup itself is found to result in a transitory suppression in the ionospheric electric field in both hemispheres. This flow suppression is very similar to that observed in HF radar observations of full substorm onset, with the exception of a lack of subsequent poleward expansion.  相似文献   

10.
Coincident multi-instrument magnetospheric and ionospheric observations have made it possible to determine the position of the ionospheric footprint of the magnetospheric cusp and to monitor its evolution over time. The data used include charged particle and magnetic field measurements from the Earth-orbiting Viking and DMSP-F7 satellites, electric field measurements from Viking, interplanetary magnetic field and plasma data from IMP-8, and Sondrestrom incoherent scatter radar observations of the ionospheric plasma density, temperature, and convection. Viking detected cusp precipitation poleward of 75.5○ invariant latitude. The ionospheric response to the observed electron precipitation was simulated using an auroral model. It predicts enhanced plasma density and elevated electron temperature in the upper E- and F-regions. Sondrestrom radar observations are in agreement with the predictions. The radar detected a cusp signature on each of five consecutive antenna elevation scans covering 1.2 h local time. The cusp appeared to be about 2○ invariant latitude wide, and its ionospheric footprint shifted equatorward by nearly 2○ during this time, possibly influenced by an overall decrease in the IMF Bz component. The radar plasma drift data and the Viking magnetic and electric field data suggest that the cusp was associated with a continuous, rather than a patchy, merging between the IMF and the geomagnetic field.  相似文献   

11.
We document the detailed dynamics of the dayside aurora in the ≈1200–1600 MLT sector in response to a sharp southward turning of the interplanetary magnetic field (IMF) under negative IMF By conditions. Features not documented in previous work are elucidated by using two meridan scanning photometers (separated by 2 h) and an all-sky auroral imager in Ny Ålesund, Svalbard (75.5^MLAT) in combination with magnetograms from stations on Svalbard, covering the latitude range 71^–75^MLAT. The initial auroral response may be divided into three phases consisting of: (1) intensification of both the red (630.0 nm) and green (557.7 nm) line emissions in the cusp aurora near 1200 MLT and ≈100 km equatorward shift of its equatorward boundary, at ≈75^MLAT, (2) eastward and poleward expansions of the cusp aurora, reaching the 1430 MLT meridian after 5–6 min, and (3) east-west expansion of the higher-latitude aurora (at ≈77^–78^MLAT) in the postnoon sector. The associated magnetic disturbance is characterized by an initial positive deflection of the X-component at stations located 100–400 km south of the aurora, corresponding to enhanced Sunward return flow associated with the merging convection cell in the post-noon sector. The sequence of partly overlapping poleward moving auroral forms (PMAFs) during the first 15 min, accompanied by corresponding pulsations in the convection current, was followed by a strong westward contraction of the cusp aurora when the ground magnetograms indicated a temporary return to the pre-onset level. These observations are discussed in relation to the Cowley-Lockwood model of ionospheric response to pulsed magnetopause reconnection.  相似文献   

12.
A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (Kp = 7- and IMF Bz <0). The ionosondes measured electron densities of up to 9 × 1011 m−3 in the patch center, an increase above the density minimum between patches by a factor of ≈4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF By > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF By) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.  相似文献   

13.
The geomagnetic and auroral response to the variations in the solar wind dynamic pressure (Pd) are investigated in the periods of positive values of the IMF B z component. It is shown that the growth of Pd results in the intensification of luminosity along the auroral oval and in the poleward expansion of the poleward boundary of luminosity (PBL) in the nightside part of the oval by ～7° in latitude at a velocity of ～0.5 km/s and is accompanied by an enhancement of the DP2-type current system. A decrease in Pd, accompanied by an abrupt reversal of the IMF B y polarity from positive to negative, results in an enhancement of the westward electrojet and in a poleward shift of PBL and electrojet center. The conclusion has been made that the available three types of auroral response to Pd variations differ in the azimuthal velocity of the luminosity region or particle precipitation along the auroral oval: V 1 ～ 30–40 km/s, V 2 ～ 10, and V 3 ～ 1 km/s.  相似文献   

14.
Auroral and airglow emissions over Eureka (89° CGM) during the 1997/98 winter show striking variations in relation to solar wind IMF changes. The period January 19 to 22, 1998, was chosen for detailed study, as the IMF was particularly strong and variable. During most of the period, Bz was northward and polar arcs were observed. Several overpasses by DMSP satellites during the four day period provided a clear picture of the particle precipitation producing the polar arcs. The spectral character of these events indicated excitation by electrons of average energy 300 to 500 eV. Only occasionally were electrons of average energy up to 1 keV observed and these appeared transitory from the ground optical data. It is noted that polar arcs appear after sudden changes in IMF By, suggesting IMF control over arc initiation. When By is positive there is arc motion from dawn to dusk, while By is negative the motion is consistently dusk to dawn. F-region (anti-sunward) convections were monitored through the period from 630.0 nm emissions. The convection speed was low (100/150 m/s) when Bz was northward but increased to 500 m/s after Bz turned southward on January 20.  相似文献   

15.
行星际磁场对极尖区位形变化的统计研究   总被引：1，自引：0，他引：1       下载免费PDF全文
利用Cluster卫星数据,选取2001—2010年期间的616个极尖区穿越事件,研究了行星际磁场（IMF）的大小和方向对极尖区位形的影响.结果表明：当Bz为北向时,随着Bx负向的增大,极尖区的磁不变纬度向高纬方向（极区）移动;当Bz为南向时,随着Bx负向增大,极尖区的磁不变纬度略微向低纬度方向（赤道）移动.Bx正向增大时,极尖区并没有明显移动.Bx对极尖区影响在南半球较为显著,在北半球没有明显规律性变化.此外,随着行星际磁场锥角的增大（>90°）,极尖区也随之向高纬移动.当Bz南向时,随着By的负向增大,极尖区在北半球向晨侧移动,在南半球向昏侧移动.而当Bz南向增加时,南北半球两个极尖区的磁不变纬度都朝赤道方向移动;但北向Bz时几乎没有移动.  相似文献   

16.
It is shown that the interaction of the interplanetary magnetic field (IMF), when it has southward component, with the geomagnetic field leads to the formation of an enhanced pressure layer (EPL) near the magnetopause. Currents flowing on the boundary between the EPL and the magnetosheath prevent the IMF from penetrating the magnetosphere. However, the outward boundary of the EPL is unstable. The interchange instability permanently destroys the EPL. Separate filaments of the EPL move away from the Earth. New colder plasma of the magnetosheath with a frozen magnetic field replaces the hotter EPL plasma, and the process of EPL formation and destruction repeats itself.The instability increment is calculated for various magnitudes of the azimuthal wave number, ky, and curvature radius of the magnetic field lines, Rc. The disturbances with R−1e\leqky\leq4R−1e (where Re is the Earth’s radius) and Rc\simeqRe are the most unstable.A possible result of the interchange instability of the EPL may be patchy reconnection, displayed as flux transfer events (FTEs) near the magnetopause.  相似文献   

17.
Observations by the EISCAT Svalbard radar in summer have revealed electron density enhancements in the magnetic noon sector under conditions of IMF Bz southward. The features were identified as possible candidates for polar-cap patches drifting anti-Sunward with the plasma flow. Supporting measurements by the EISCAT mainland radar, the CUTLASS radar and DMSP satellites, in a multi-instrument study, suggested that the origin of the structures lay upstream at lower latitudes, with the modulation in density being attributed to variability in soft-particle precipitation in the cusp region. It is proposed that the variations in precipitation may be linked to changes in the location of the reconnection site at the magnetopause, which in turn results in changes in the energy distribution of the precipitating particles.  相似文献   

18.
Results are presented from a multi-instrument investigation of the signatures of equatorial reconnection in the summer, sunlit ionosphere. Well-established ion dispersion signatures measured during three DMSP satellite passes were used to identify footprints in ionospheric observations made by radio tomography, and both the EISCAT ESR and mainland radars. Under the prevalent conditions of southward IMF with the Bz component increasing in magnitude, the reconnection footprint was seen to move equatorward through the ESR field-of-view. The most striking signature was in the electron temperatures of the F2 region measured by the EISCAT mainland radar that revealed significantly enhanced temperatures with a steep equatorward edge, in general agreement with the leading edge of the ion dispersion. It is suggested that this sharp transition in the electron temperature may be an indicator of the boundary, mapping from the reconnection site, between closed geomagnetic field lines and those opened along which magnetosheath ions precipitate.  相似文献   

19.
We present data from conjugate SuperDARN radars describing the high-latitude ionospheres response to changes in the direction of IMF By during a period of steady IMF Bz southward and Bx positive. During this interval, the radars were operating in a special mode which gave high-time resolution data (30 s sampling period) on three adjacent beams with a full scan every 3 min. The location of the radars around magnetic local noon at the time of the event allowed detailed observations of the variations in the ionospheric convection patterns close to the cusp region as IMF By varied. A significant time delay was observed in the ionospheric response to the IMF By changes between the two hemispheres. This is explained as being partially a consequence of the location of the dominant merging region on the magnetopause, which is 8/12RE closer to the northern ionosphere than to the southern ionosphere (along the magnetic field line) due to the dipole tilt of the magnetosphere and the orientation of the IMF. This interpretation supports the anti-parallel merging hypothesis and highlights the importance of the IMF Bx component in solar wind-magnetosphere coupling.  相似文献   

20.
The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative Bz and positive By, to positive Bz and small By, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within 2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of 7 and 14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further 6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.  相似文献   

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