A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere

First results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E × B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E × B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wave.     

Interplanetary magnetic field By control of dayside auroras

The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.     

Comparing the latitudinal distribution of the <Emphasis Type="Italic">Pc</Emphasis>1 intensity and the position of subauroral proton spots

The latitudinal position of subauroral proton spots (special proton auroras observed from the IMAGE satellite) has been compared with the Pc1 pulsation intensity distribution determined using the data from the Finnish meridional network of induction magnetometers. It has been indicated that a Pc1 intensity maximum is always observed at the station that is closer to the proton aurora projection. Two Pc1 bands were registered in the event when two proton auroral spots were simultaneously observed at different latitudes. In this case, the Pc1 intensity distribution maximum at lower frequencies was related to a proton auroral spot at a higher latitude and vice versa. Such a spatial correlation between Pc1 pulsations and proton auroral spots, together with the previously established time correlation between these phenomena, demonstrates that subauroral proton spots reflect the region of ion cyclotron instability in the equatorial magnetosphere at the level of the ionosphere.     

The association between giant pulsations (Pgs) and the auroral oval

Two features of giant pulsations (Pgs) which still require an explanation are firstly, why Pgs occur mainly in the early morning sector (i.e. 03:00-07:00 MLT) and not at other times of day, and secondly, why Pgs occur preferentially in a narrow latitudinal band (approximately 63○-68○ geomagnetic latitude). Using statistics from 34 Pg events observed by the EISCAT magnetometer cross, a comparison has been made between the location of the Pg resonant field lines and the equatorward edge of the auroral oval. The majority of these Pg events appear to occur just poleward of this boundary. Using these results, an explanation of the two features of Pgs as detailed above is made. This explanation involves the interaction of protons, which may be responsible for the Pg events, with the inner edge of the plasma sheet or with its ionospheric equivalent, the equatorward edge of the auroral oval.     

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external influences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric fields play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions affect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual effect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the effect of the composition changes is modified by the large solar zenith angle at midwinter. The zenith angle effect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle effect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/molecular ratio. This leads to a semiannual variation of NmF2. A different situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.     

Thermospheric response to ion heating in the dayside cusp

Recent observations from the CHAMP satellite indicate that neutral density enhancements are common in the northern dayside cusp. The neutral density in this region can be nearly a factor of 2 larger than in adjacent regions of the thermosphere on the poleward and equatorward sides of the cusp. The presence of density enhancements implies that the neutral atmosphere is being heated in the cusp region causing upwelling. A high-resolution model of the global thermosphere is used to study the thermospheric response to heating in the northern dayside cusp. It is found that heating in the cusp results in the creation of a neutral fountain. Specifically, upward drift of the thermosphere within the cusp region is followed at higher altitudes by poleward and equatorward movement out of the cusp region and the gradual subsidence of the neutral gas. Density enhancements of the magnitude observed by the CHAMP satellite occur in the model results for sufficiently strong heating in the cusp. Neutral temperature enhancements also occur and are strongest near the poleward and equatorward boundaries of the cusp region.     

基于行星际/太阳风和地磁条件的紫外极光卵边界建模和预测

极光卵的尺度大小与太阳风-磁层-电离层能量耦合过程紧密相关,准确预测其大小对空间天气研究和预报具有非常重要的意义.本文基于模糊c均值聚类算法,从Polar卫星紫外极光图像中自动提取极光卵边界数据(~1215000个赤道向边界点和~3805000极向边界点),统计分析其与太阳风等离子体、行星际磁场、地磁指数等之间的相关特性,并构建了以行星际、太阳风为模型参数(模型1)和以行星际、太阳风及地磁指数为模型参数(模型2)的2种极光卵边界多元回归模型.以模型预测的极光卵边界与实际极光卵边界之间的平均绝对误差作为模型评价标准,将本文预测模型与Carbary(2005)模型和Milan(2009)模型进行了对比.结果表明,模型2对极光卵极向、赤道向边界预测的平均绝对误差为1.55和1.66地磁纬度,优于Carbary和Milan模型(Carbary模型极向、赤道向边界的平均绝对误差为2.18和5.47地磁纬度,Milan模型极向、赤道向边界的平均绝对误差为1.71地磁纬度和1.90地磁纬度).     

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m⁻¹) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.     

Auroral intensification structure and dynamics in the double oval: Substorm of December 26, 2000

Based on results of the simultaneous TV observations at Barentsburg high-latitude observatory and Lovozero auroral observatory and using the IMAGE auroral luminosity images, the auroral fine structure and dynamics has been studied during the substorm of December 26, 2000, when the auroral luminosity distribution represented a double oval. It has been indicated that the interaction between the processes proceeding in different magnetospheric regions, the projections of which are the poleward and equatorward edges of the double oval, is observed in auroras in the process of substorm development.     

Position of projections of the nightside auroral oval equatorward and poleward edges in the magnetosphere equatorial plane

The position of the auroral oval poleward and equatorward boundary projections on the equatorial plane in the nightside MLT sector during magnetically quiet periods (|AL| < 200 nT, |Dst| < 10 nT) has been determined. The oval boundary positions were determined according to the precipitation model developed at Polar Geophysical Institute (http://apm.pgia.ru/). The isotropy of the averaged plasma pressure and the experimentally confirmed balance of pressures during the nighttime have been taken into account. The morphological mapping method has been used to map the oval poleward and equatorward edges without the use of any magnetic field model on the assumption that the condition of magnetostatic equilibrium is valid. Ion pressures at ionospheric altitudes and in the equatorial plane have been compared. It has been shown that the auroral oval equatorward boundary in the midnight sector is localized at geocentric distances of ~7 R_E, which is in good agreement with the position of the energetic particle injection boundary in the equatorial plane. The oval poleward edge is localized at the ~10 R_E geocentric distance, which is in good agreement with the position of the equatorward boundary of the region with a high turbulence level in the Earth’s magnetosphere plasma sheet.     

On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

Observations and modelling are presented which illustrate the ability of the Finland CUTLASS HF radar to monitor the afternoon-evening equatorward auroral boundary during weak geomagnetic activity. The subsequent substorm growth phase development was also observed in the late evening sector as a natural continuation of the preceding auroral oval dynamics. Over an 8 h period the CUTLASS Finland radar observed a narrow (in range) and persistent region of auroral F- and (later) E-layer echoes which gradually moved equatorward, consistent with the auroral oval diurnal rotation. This echo region corresponds to the subvisual equatorward edge of the diffuse luminosity belt (SEEL) and the ionospheric footprint of the inner boundary of the electron plasma sheet. The capability of the Finland CUTLASS radar to monitor the E-layer SEEL-echoes is a consequence of the nearly zero E-layer rectilinear aspect angles in a region 5/10° poleward of the radar site. The F-layer echoes are probably the boundary blob echoes. The UHF EISCAT radar was in operation and observed a similar subvisual auroral arc and an F-layer electron density enhancement when it appeared in its antenna beam.     

Upper-atmospheric effects of magnetic storms: a brief tutorial

The physical processes underlying several phenomena of upper-atmospheric storms are described: magnetospherically driven ion convection and Joule heating and their impact on the high-latitude thermosphere and ionosphere; global changes in thermospheric circulation and composition; traveling atmospheric disturbances; and effects of electric-field penetration to middle and low latitudes. Examples from the 1997 January 10–11 storm are used to illustrate some of these features. It is pointed out that not only the magnitude, but also the sign of many storm-time changes at any given location depend sensitively on the temporal and spatial variations of auroral particle precipitation and high-latitude electric fields. In order for simulation models to be able to predict upper-atmospheric storm effects accurately, improved determination of the high-latitude inputs will be required.     

Auroral precipitation dynamics during strong magnetic storms

The dynamics of the auroral precipitation boundaries in the daytime (0900–1200 MLT) and nighttime (2100–2400 MLT) sectors during two strong magnetic storms of February 8–9, 1986, and March 13–14, 1989, with a Dst value at a maximum of approximately ?300 and ?600 nT, respectively, are studied using the DMSP satellite data. It is shown that, during the main phase of a storm, a shift to lower latitudes of the poleward and equator ward boundaries of the daytime precipitation is observed. In the nighttime sector, the equatorward boundary of the precipitation also shifts to lower latitudes, whereas the position of the poleward boundary depends weakly on the magnetic activity level even in the periods of very strong magnetic disturbances. The increase in the polar cap area occurs mainly due to the equatorward shift of the daytime precipitation. A high correlation degree between the equatorward shift of the poleward boundary of the daytime precipitation and the position of the equatorward boundary of the precipitation at the nighttime side of the Earth is demonstrated. The analysis of the events shows that (1) the magnetic activity level in the nighttime sector of the auroral zone influences considerably the position of the daytime precipitation boundaries during magnetic storms and that (2) the ring current inputs considerably into the value of the Dst variations.     

Observations of thermospheric neutral winds within the polar cusp and the auroral oval using a Doppler imaging system (DIS)

Two Doppler imaging systems (DIS) or wide-field imaging Fabry-Perot interferometers (FPI), have recently been commissioned, one at the Auroral Station, Adventdalen, Longyearbyen, Svalbard, and the second at the IRF, Kiruna, Sweden. These instruments can provide wide-field (600 * 800 km) images of neutral wind flows in the upper thermosphere, by measuring the Doppler shift of the atomic oxygen forbidden near 630 nm, which is emitted from an altitude of approximately 240 km. From the instrument in Svalbard, at mid-winter, it is possible to observe the dayside polar cusp and the polar cap throughout the entire day, whereas from Kiruna, the night-time auroral oval is observable during the hours of darkness. Measurements of thermospheric dynamics from the DIS can be used in conjunction with observations of ionospheric plasma flows and thermal plasma densities by the EISCAT-Svalbard radar (ESR) and by EISCAT, along with other complementary observations by co-located instruments such as the auroral large-scale imaging system (ALIS). Such combined data sets will allow a wide range of scientific studies to be performed concerning the dynamical response of the thermosphere and ionosphere, and the important energetic and momentum exchange processes resulting from their complex interactions. These processes are particularly important in the immediate vicinity of the polar cusp and within the auroral oval. Early results from Svalbard in late 1995 will be discussed. The DIS in Kiruna observed two interesting geomagnetic disturbances in early 1997, the minor geomagnetic storm of 10, 11 January, and the disturbed period from 7–10 February. During these events, the thermospheric wind response showed some interesting departures from the average behaviour, which we attribute to the result of strong and variable Lorenz forcing (ion drag) and Joule and particle heating during these geomagnetic disturbances.     

Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles

According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of > 30-keV electrons, whereas (2) the IB of the > 30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18– 01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines.     

Causes of <Emphasis Type="Italic">NmF</Emphasis>2 longitudinal variations at mid- and subauroral latitudes under summer nighttime conditions

The main factors controlling NmF2 longitudinal variations at mid- and subauroral latitudes have been studied. The data of the Intercosmos-19 topside sounding, obtained at high solar activity for summer nighttime conditions, have been used in the analysis. The contributions of the solar ionization, neutral wind, and temperature and composition of the thermosphere to NmF2 longitudinal variations have been estimated based on ionospheric models. It has been indicated that NmF2 variations in the unsunlit midlatitude ionosphere mainly depends on the residual electron density and its decay under the action of recombination. At subauroral latitudes under summer nighttime conditions, the ionosphere is partially sunlit, and ionization by solar radiation mainly contributes to NmF2 longitudinal variations, whereas the effect of the neutral wind is slightly less significant. These results also indicate how the contribution of different factors to NmF2 longitudinal variations changes at different latitudes.     

Relationship between auroral oval poleward boundary intensifications and magnetic field variations in the solar wind

As a rule, bright auroral arcs evolve near the poleward boundary of the auroral oval at the growth phase of a substorm, a phenomenon that is known to occur near the poleward edge of the auroral oval. The closeness of these arcs to the projection of the magnetic separatrix on the night side suggests that their generation is related to magnetic reconnection in the magnetospheric tail in a particular way. In this study this suggestion is confirmed by the fact that integral brightness of the auroral oval at the poleward edge correlates with magnetic field structures in the solar wind that are observed by ACE and Wind satellites at distances of 50–300 R_E upstream and are shifted towards the magnetospheric tail with time delays of ~ 10–80 min, consistent with measurements of the solar wind velocity. About 50 examples of this correlation have been found. The possible physical mechanisms of the effect observed are discussed.     

A large vertical wind in the thermosphere at the auroral oval/polar cap boundary seen simultaneously from Mawson and Davis,Antarctica

Ground-based Fabry–Perot spectrometer observations from the Australian Antarctic stations of Davis and Mawson show an upward wind ≥100 m s⁻¹ in the thermosphere at ∼240 km altitude on the night of Day of Year 159 in 1997. The wind was from a region located poleward of the poleward edge of the discrete auroral oval, and is identified as a further event of the type seen at Mawson, and elsewhere, in earlier work. The upward wind was first seen over Davis station at ∼22:00 UT. As the auroral oval moved northward the region of upward wind followed, and was seen at Mawson (some 4° magnetically north of Davis) just over 1 h later. It is shown that the presence of the large upward wind does, at times, affect the horizontal wind inferred from the off-zenith observations. Correcting the affected measurements for the non-zero upward wind leads to a horizontal wind field more consistent with that derived from observations before and after the vertical wind event. A lower limit of the area of the region of upward wind over Mawson and Davis on this night is estimated as ∼6×10¹¹ m². The estimated power required to drive the upward wind over this area at 240 km altitude is of order 6×10⁹ W. We estimate that this represents between 3 and 7% of the geomagnetic power input in the southern hemisphere during this interval.