Magnetic storm effects in the auroral ionosphere observed with EISCAT radar—Two case studies

Storm-time changes of main plasma parameters in the auroral ionosphere are analyzed for two intense storms occurring on May 15, 1997 and Sept. 25, 1998, with emphasis on their relationship to the solar wind dynamic pressure and the IMFB _z component. Strong hard particle precipitation occurred in the initial phase for both storms, associated with high solar wind dynamical pressure. During the recovery phase of the storms, some strong particle precipitation was neither concerned with high solar wind pressure nor southward IMFB _z. Severe negative storm effects depicted by electron density depletion appeared in theF-region during the main and recovery phase of both storms, caused by intensive electric field-related strong Joule/frictional heating when IMF was largely southward. The ion temperature behaved similarly inE-andF-region, but the electron temperature did quite different, with a strong increase in the lowerE-region relating to plasma instability excited by strong electric field and a slight decrease in theF-region probably concerning with a cooling process. The field-aligned ion velocity was high and apparently anticorrelated with the northward component of the ion convection velocity. Foundation item: Supported by the National Natural Science Foundation of China (49674241) and the Research Fund for the Doctoral Program of Higher Education Biography: LIU Hui-xin (1974-), female, Ph.D. candidate. Research direction: polar ionospheric behaviors during magnetic storms.     

基于无线电掩星观测的火星电离层观测研究进展

火星电离层早期的观测数据非常少,除了Viking登陆器对火星电离层的在位测量外,火星电离层的主要物理信息是通过掩星观测方法得到的.近年来,Mars Global Surveyor和Mars Express轨道器通过掩星观测的方法对火星的上层大气和电离层进行了长期的观测,得到了大量的火星电离层电子密度廓线资料.火星电离层受到来自太阳EUV和X射线辐射、太阳风、太阳耀斑、中性大气、表面壳磁场、宇宙射线、流星等多种因素的影响,使其结构发生瞬态或季节性的变化.本文介绍了行星无线电掩星探测的基本原理和技术特点,回顾了国内外科学家们基于已有的火星掩星观测数据（主要是Mars GlobalSurveyor和Mars Express）在火星电离层研究中的一些最新科学成果,并详细介绍了火星电离层的结构和火星夜间电离层的主要特征.     

Magnetic Storm Effects in the Auroral Ionosphere Observed with EISCAT Radar -Two Case Studies

Storm-time changes of main plasma parameters in the auroral ionosphere are analyzed for two intense storms occurring on May 15, 1997 and Sept. 25, 1998, with emphasis on their relationship to the solar wind dynamic pressure and the IMF Bz component. Strong hard particle precipitation occurred in the initial phase for both storma,associated with high solar wind dynamical pressure. During the recovery phase of the storms, some strong particle precipitation was neither concerned with high solar wind pressure nor southward IMF Bz. Severe negative storm effects depicted by electron density depletion appeared in theF-region during the main and recovery phase of both storms, caused by intensive electric field-related strong Joule/frictional heating when IMF was largely southward. The ion temperature behaved similarly in E- and F-region, but the electron temperature did quite different, with a strong increase in the lower E-region relating to plasma instability excited by strong electric field and a slight decrease in the F-region probably concerning with a cooling process. The field-aligned ion velocity was high and apparently anticorrelated with the northward component of the ion convection velocity.     

中性风对暴时电离层影响的研究

从电离层等离子体动力学方程出发,研究了磁暴期间中性风对中低纬度电离层的影响.通过计算机模拟发现:中性风能够对暴时电离层等离子体的垂直向上漂移产生抑制作用,并延迟电离层F层底部"抛空"的形成;不论采用哪种中性风模式,磁暴都会对较低纬度的电离层产生更大的影响.     

The evidence for the evolution of interplanetary small flux ropes: Boundary layers

We have examined the Wind data in 1996 and identified 21 small interplanetary magnetic flux ropes(SIMFRs),and all the 21 SIMFRs have boundary layer structures.The durations of the boundary layers varied from several minutes to 30 minutes.These boundary layers also have properties of high proton temperature,density,and plasma beta.These boundary layers are formed by magnetic reconnections.In addition,in three events magnetic reconnections were occurring inside the boundary layers.It indicates that the flux rope structures have propagated for some period of time,and their boundaries were still evolving through interaction with the background solar wind.Namely it is very possible that the SIMFRs came from the solar corona.     

Solar activity effects of the ionosphere: A brief review

Solar radiation, which varies over multiple temporal scales, modulates remarkably the evolution of the ionosphere. The solar activity dependence of the ionosphere is a key and fundamental issue in ionospheric physics, providing information essential to understanding the variations in the ionosphere and its processes. Selected recent studies on solar activity effects of the ionosphere are briefly reviewed in this report. This report focuses on (1) observations of solar irradiance at X-ray and extreme ultraviolet wavelengths and the outstanding problems of solar proxies, in the view of ionospheric studies, (2) new findings and improved representations of the features of the solar activity dependence of ionospheric key parameters and the corresponding physical processes, (3) possible phenomena in the ionosphere under extremely high and low solar activity conditions that are unique, as indicated by historical solar datasets and the deep solar minimum of solar cycle 23/24, and (4) statistical studies and model simulations of the ionosphere response to solar flares. The above-mentioned studies provide new clues for comprehensively explaining basic processes in the ionosphere and improving the prediction capability of ionospheric models and related applications.     

Little or no solar wind enters Venus' atmosphere at solar minimum

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.     

Numerical simulation of ionization enhancement in the topside ionosphere of cusp foot-point region caused by low energy particle precipitation

The Graz Ionospheric Flux Tube Simulations (GIFTS) has been improved. The improved GIFTS model was used to numerically investigate the energy particle precipitation on the distribution of electron density in the ionospheric cusp foot-point region under conditions of large plasma convection during magnetic storm. After including the effects of low energy incident particles, the ionospheric electron densities increase remarkably above altitude of ∼250 km, showing a peak at about 350 km. The percent enhancements of electron densities increase gradually with altitude, exceeding 60% near the upper boundary of the calculation. The calculated ionospheric F ₂-peak was remarkably enhanced and lifted up by the incident low energy electrons. Biography: CAI Hongtao(1976–), male, Associate professor, Ph.D., research direction: polar magnetosphere and ionosphere.     

Nonlinear dispersive scale Alfvén waves inmagnetosphere-ionosphere coupling: Physical processes and simulation results

Dispersive Alfvén waves(DAWs)have been demonstrated to play a significant role in auroral generation of the magnetosphereionosphere coupling system.Starting from a two fluid reduced MHD model,we summarize the frequency,temporal and spatial characteristics of magnetospheric DAWs.Then,the nonlinear kinetic and inertial scale Alfveén waves are studied,and we review some theoretical aspects and simulation results of dispersive Alfve′n waves in Earth’s magnetosphere.It is shown that dispersive standing Alfve′n waves can generate the field-aligned currents which transport energy into the auroral ionosphere,where it is dissipated by Joule heating and energy lost due to electron precipitation.The Joule dissipation can heat the ionospheric electron and produce changes in the ionospheric Pedersen conductivity.As a feedback,the conducting ionosphere can also strongly affect the magnetospheric currents. The ponderomotive force can cause the plasma to move along the field line,and generate ionospheric density cavity.The nonlinear structuring can lead to a dispersive scale to accelerate auroral particle,and the Alfvn waves can be trapped within the density cavity. Finally,we show the nonlinear decay of dispersive Alfvén waves related to two anti-propagating electron fluxes observed in the auroral zone.     

Solar wind dynamic pressure and electric field as the main factors controlling Saturn's aurorae

The interaction of the solar wind with Earth's magnetosphere gives rise to the bright polar aurorae and to geomagnetic storms, but the relation between the solar wind and the dynamics of the outer planets' magnetospheres is poorly understood. Jupiter's magnetospheric dynamics and aurorae are dominated by processes internal to the jovian system, whereas Saturn's magnetosphere has generally been considered to have both internal and solar-wind-driven processes. This hypothesis, however, is tentative because of limited simultaneous solar wind and magnetospheric measurements. Here we report solar wind measurements, immediately upstream of Saturn, over a one-month period. When combined with simultaneous ultraviolet imaging we find that, unlike Jupiter, Saturn's aurorae respond strongly to solar wind conditions. But in contrast to Earth, the main controlling factor appears to be solar wind dynamic pressure and electric field, with the orientation of the interplanetary magnetic field playing a much more limited role. Saturn's magnetosphere is, therefore, strongly driven by the solar wind, but the solar wind conditions that drive it differ from those that drive the Earth's magnetosphere.     

Response of the magnetic field and plasmas at the geosynchronous orbit to interplanetary shock

Interplanetary shock can greatly disturb the Earth＇s magnetosphere and ionosphere, causing the temporal and spatial changes of the magnetic field and plasmas at the geosynchronous orbit. In this paper, we use the magnetic field data of GOES satellites from 1997 to 2007 and the plasma data of MPA on the LANL satellites from 1997 to 2004 to study the properties of magnetic field and plasma （0.03-45 keV） at the geosynchronous orbit （6.6 RE） within 3 hours before and after the arrival of shock front at the geosynchronous orbit through both case study and superposed epoch analysis. It is found that following the arrival of shock front at the geosynchronous orbit, the magnetic field magnitude, as well as GSM Bzcomponent increases significantly on the dayside （8-16 LT）, while the By component has almost no change before and after shock impacts. In response to the interplanetary shock, the proton becomes much denser with a peak number density of 1.2 cm^-3, compared to the typical number density of 0.7 cm^-3. The proton temperature increases sharply, predominantly on the dusk and night side. The electron, density increases dramatically on the night side with a peak number density of 2.0 cm^-3. The inferred ionospheric O^＋ density after the interplanetary shock impact reaches the maximum value of 1.2 cm^-3 on the dusk side and exhibits the clear dawn-dusk asymmetry. The peak of the anisotropy of proton＇s temperature is located at the noon sector, and the anisotropy decreases towards the dawn and dusk side. The minimum of temperature anisotropy is on the night side. It is suggested that the electromagnetic ion cyclotron （EMIC） wave and whistler wave can be stimulated by the proton and electron temperature anisotropy respectively. The computed electromagnetic ion cyclotron wave （EMIC） intense on the day side （8--16 LT） with a frequency value of 0.8 Hz, and the wave intensity decreases towards the dawn and dusk side, the minimum value can be found on the night side. The computed electron whistler wave locates on the day side （8--16 LT） with a value of 2 kHz.     

Multi-scale pressure-balanced structures in the solar wind observed by WIND

This work detects multi-scale, from hour to seconds, pressure-balanced structures (PBSs) in the solar wind based on the anti- correlation between the plasma thermal pressure and the magnetic pressure measured by WIND at 1 AU on April 5th, 2001. In our former research based on Cluster measurements, we showed the anti-correlation between the electron density and the magnetic field strength in multi-scales, and we supposed these structures may be pressure-balanced structures. Thus, in this work we aim to prove our speculation by the direct evidence on pressure measurements. Different from our previous work, we apply the WIND measurements this time, for they have both the magnetic pressure and the plasma pressure which Cluster could not offer. We use the wavelet cross-coherence method to analyze the correlation between the plasma pressure (P th ) and the magnetic pressure (P B ), and also the electron density (N e ) and the magnetic field strength (B) on various scales. We observe the anti-correlation between P th and P B distributed at different temporal scales ranging from 1000 s down to 10 s. This result directly indicates the existence of pressure- balanced structures (PBSs) with different sizes in the solar wind. Further, We compare the wavelet cross correlation spectrum of P th -P B and N e -B. We notice that the two spectra are similar in general. Thus this result confirms that the relation between P th -P B and N e -B are consistent with each other in the PBSs we study. Moreover, we compare the power spectrum density (PSD) of relative N e fluctuation with our previous work based on Cluster measurements. The two spectra show similar trend with Komolgorov’s -5/3 as their slopes. This may imply the similarity of the structures observed by both WIND and Cluster spacecrafts. Finally, we discuss the possible formation mechanisms for these multi-scale pressure-balanced structures. Our result is important to support the existence of multi-scale PBSs from one-hour scale down to one-minute, and is helpful to understand the role of compressive fluctuation in the solar wind turbulence dominated by Alfvénic cascading.     

Comparison of a new model with previous models for the low-latitude magnetopause size and shape

In this study, the advantages and the limitations of previous low-latitude magnetopause empirical models are discussed. In order to overcome their limitations and inherit their advantages, a new continuous function for the influence of the interplanetary magnetic field (IMF) Bz on the magnetopause, the Shue model function and the 613 low-latitude magnetopause crossings are used to construct a new low-latitude magnetopause model parameterized by the solar wind dynamic pressure (Dp) and IMF Bz. In comparison ...     

Analysis of ionospheric anomalies before the 2011 <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> 9.0 Japan earthquake

On March 11, 2011, a large earthquake of Mw=9.0 occurred near the east coast of Honshu, Japan. This paper investigates preearthquake ionospheric anomalies during the earthquake period, using data from global navigation satellite systems and ionosonde stations near the epicenter. A clear anomaly that occurred on March 8 lasted 6 hours. Eliminating ionospheric anomalies that may have been caused by solar activities and magnetic storms, we believe that a positive anomaly on March 8 was very possibly an ionospheric precursor. The affected ionospheric area on March 8, which is evident on a vertical total electron content distribution map, extended to 50° in longitude and 20° in latitude, with length ratio approximately 3:1. The anomaly peak arose from 15:00-19:00 LT, and its location did not coincide with the vertical projection of the epicenter, but was instead to its south. Corresponding ionospheric anomalies are also observed in the magnetically conjugated region. There were no obvious ionospheric anomalies in other parts of the world. To analyze changes in the ionospheric anomaly, computerized ionospheric tomography technology was used to invert the spatial and temporal distribution of electron density in the ionosphere. The ionospheric anomaly on March 8, 2011 is suggested to be an ionospheric precursor of the March 11 earthquake in Japan.     

Enhanced anti-sunward flow near local noon during a period of horizontal IMF and high solar wind velocity <Emphasis Type="Italic">V</Emphasis><Subscript><Emphasis Type="Italic">Y</Emphasis></Subscript>

It is believed that a southward interplanetary magnetic field (IMF) is mainly responsible for the energy input from solar wind into the magnetosphere. This paper presents an unusual case of strong anti-sunward plasma flow (up to 2 km/s) in the polar cap ionosphere and large cross-polar cap potential (CPCP) during a period of horizontal IMF (|B _Z | < 2 nT) observed by both ACE (at the L1 point) and Geotail (on the dusk flank of the magnetosheath). The CPCP is even higher than that under preceding B _Z ≈ −23 nT. Furthermore, GOES8 observed that the magnetosheath field turns northward as the anti-sunward plasma flow and CPCP start to increase, which implies that the magnetosheath field interacting with the Earth’s magnetopause has significantly rotated and differs from the IMF observed by ACE and Geotail. In accordance with previous theoretical work, we suggest that the magnetic field line draping produces a southward magnetosheath field and enhances anti-sunward plasma flow and the CPCP.     

Dependence of magnetic field just inside the magnetopause on subsolar standoff distance: Global MHD results

The subsolar magnetopause is the boundary between the solar wind and the Earth’s magnetosphere,where reduced solar wind dynamic pressure is equal to the magnetic pressure of the Earth’s outer magnetosphere.We use a global magnetohydrodynamic (MHD)model to estimate the ratio f of the compressed magnetic field just inside the subsolar magnetopause to the purely dipolar magnetic field.We also compare our numerical results to a similar work by Shue,which used Time History of Events and Macroscale Interactions during Substorms(THEMIS)data.Our results show that the ratio f is linearly proportional to the subsolar magnetopause standoff distance(r0)for both the northward and southward interplanetary magnetic field,properties consistent with Shue but with a smaller proportionality constant.However,previous theoretical studies show that f is nearly independent of the subsolar standoff distance.The global model results also show that f is smaller for the southward Interplanetary Magnetic Field(IMF)under the same r0,and that the proportionality constant for the southward IMF is larger than that for the northward IMF.Both conclusions agree with statistical results from observations by Shue.     

Responses of properties in the plasma sheet and at the geosynchronous orbit to interplanetary shock

On July 22, 2004, the WIND spacecraft detected a typical interplanetary shock. There was sustaining weak southward magnetic field in the preshock region and the southward field was suddenly enhanced across the shock front （i.e., southward turning）. When the shock impinged on the magnetosphere, the magnetospheric plasma convection was abruptly enhanced in the central plasma sheet, which was directly observed by both the TC-1 and Cluster spacecraft located in different regions. Simultaneously, the Cluster spacecraft observed that the dawn-to-dusk electric field was abruptly enhanced. The variations of the magnetic field observed by TC-1, Cluster, GOES-10 and GOES-12 that were distributed in different regions in the plasma sheet and at the geosynchronous orbit are obviously distinct. TC-1 observations showed that the magnetic intensity kept almost unchanged and the elevation angle decreased, but the Cluster spacecraft, which was also in the plasma sheet and was further from the equator, observed that the magnetic field was obviously enhanced. Simultaneously, GOES-12 located near the midnight observed that the magnetic intensity sharply increased and the elevation angle decreased, but GOES-10 located in the dawn side observed that the magnetic field was merely compressed with its three components all sharply increasing. Furthermore, the energetic proton and electron fluxes at nearly all channels observed by five LANL satellites located at different magnetic local times （MLTs） all showed impulsive enhancements due to the compression of the shock. The responses of the energetic particles were much evident on the dayside than those on the nightside. Especially the responses near the midnight were rather weak. In this paper, the possible reasonable physical explanation to above observations is also discussed. All the shock-induced responses are the joint effects of the solar wind dynamic pressure pulse and the magnetic field southward turning.     

The correlations of ions density with geomagnetic activity and solar dynamic pressure in cusp region

A statistical study of the properties of ions (O , He and H ) measured by the Cluster-II in cusp region as a function of the solar wind dynamic pressure and geomagnetic index Kp respectively was made dur-ing the summer and fall of 2001-2003. The main results are that: (1) O ion density responds in a sig-nificant way to geomagnetic index Kp, and He ion density is not correlated with geomagnetic index Kp, both of them have a significant positive correlation with solar wind dynamic pressure; (2) H ion density is also observed to increase with solar wind dynamic pressure, and not correlated with geomagnetic index Kp.     

Lightning-induced intensification of the ionospheric sporadic E layer

A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s. Several mechanisms have been proposed to explain this connection, and evidence from modelling as well as various types of measurements demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events, that the ionospheric 'sporadic E' layer--transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation--can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.     

HF waves heating ionosphere F-layer

To predict and evaluate the effects of ionospheric modification quantitatively, the electron temperature and density perturbations are analyzed at different heating conditions, based on a developed model including momentum equation, continuity equation and energy equation. The results show that (1) powerful HF radio waves can cause the electron temperature to increase markedly, and the largest perturbation is near the reflection layer, but the electron density decreases at this area; (2) the lower neutral gas density in the ionosphere, the larger disturbance of the electron temperature and density; the higher radio frequency, the lower increment of electron temperature; and the higher radio power, the larger disturbance of electron temperature and density are caused, but there are no linear relationships between the radio power and disturbance amplitude; (3) the perturbations at nighttime are larger than at daytime when heated by HF radio waves with the same powers; the electron temperature perturbations in the solar minimum are larger than that in solar maximum; and the electron temperature perturbations in spring and autumn are larger than in winter and summer; (4) compared with the low-latitude, the mid-latitude has the smaller perturbations, the maximum electron temperature perturbation is ~30% (at daytime of winter), and the density is ~5%.