Low latitude ionospheric scintillation and zonal irregularity drifts observed with GPS-SCINDA system and closely spaced VHF receivers in Kenya

In this study we have used VHF and GPS-SCINDA receivers located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya, to investigate the ionospheric scintillation and zonal drift irregularities of a few hundred meter-scale irregularities associated with equatorial plasma density bubbles for the period 2011. From simultaneous observations of amplitude scintillation at VHF and L-band frequencies, it is evident that the scintillation activity is higher during the post sunset hours of the equinoctial months than at the solstice. While it is noted that there is practically no signatures of the L-band scintillation in solstice months (June, July, December, January) and after midnight, VHF scintillation does occur in the solstice months and show post midnight activity through all the seasons. VHF scintillation is characterized by long duration of activity and slow fading that lasts till early morning hours (05:00 LT). Equinoctial asymmetry in scintillation occurs with higher occurrence in March–April than in September–October. The occurrence of post midnight VHF scintillation in this region is unusual and suggests some mechanisms for the formation of scintillation structure that might not be clearly understood. Zonal drift velocities of irregularities were measured using cross-correlation analysis with time series of the VHF scintillation structure from two closely spaced antennas. Statistical analyses of the distribution of zonal drift velocities after sunset hours indicate that the range of the velocities is 30–160 m/s. This is the first analysis of the zonal plasma drift velocity over this region. Based on these results we suggest that the east–west component of the plasma drift velocity may be related to the evolution of plasma bubble irregularities caused by the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocities and scintillation could be attributed to the asymmetry of neutral winds in the thermosphere that drives the eastward electric fields.     

Formation mechanism of great positive TEC disturbances prior to Wenchuan earthquake on May 12, 2008

The problems of physical explanation and possible mechanisms of the seismo-ionospheric effects formation are under discussion now. There are proposed different mechanisms of such effects, for example, large- and small-scale internal gravity waves (IGWs), atmospheric electric field, electromagnetic fields and emissions. However, the appearance of local large-scale seismo-ionospheric anomalies in Total Electron Content (TEC) is possible to explain only by two mechanisms: an atmospheric electric field and/or small-scale IGWs. In this paper, the simulation results for reproduction of the observed seismo-ionospheric great positive effects in TEC prior to strong Wenchuan earthquake are presented. The obtained results confirm the proposed mechanism of seismo-ionospheric effects formation by the penetration of the seismogenic electric field from the atmosphere into the ionosphere. It is suggested that so great TEC enhancement observed 3 days prior to Wenchuan earthquake could be explained by combined action of seismogenic vertical electric field and IGWs generated by the solar terminator.     

Consequences of the application of the streamer fluid model to the study of the sprite inception mechanism

We present the results of a streamer-fluid model used to investigate the electrodynamical coupling between the troposphere and upper atmosphere due to the penetration of lightning electric fields into the mesosphere and the lower ionosphere, generating sprites. The model solves the continuity equation for electrons and ions coupled to Poisson equation. The dominant physical response of the atmosphere is the formation of a screening-ionization wave. The wave shields the atmosphere above it from the action of the lightning field and, together with the conductivity reduction below it due to attachment, the wave amplifies the total field below it, allowing for the penetration of intense electric fields in the mesosphere as it propagates downwards into regions of higher density that compress the wave. This is the key physical mechanism for sprite inception. We evaluated the effects of the thundercloud charge geometry, lightning current waveshape, atmospheric conductivity, via different electron density profiles, and the effect of ionization, attachment and electron mobility coefficients in the electrical breakdown process, related to halo production, and sprite streamer initiation. The results showed that electrons with higher mobility are more efficient in shielding the lightning electric field before breakdown, causing delay, and they contribute to the formation of the streamer seed after breakdown, anticipating the sprite streamer inception. Similarly, a higher effective ionization rate, produced by modifications in the attachment and ionization coefficients, anticipates sprite inception. The simulations with 6 different electron density profiles, and therefore conductivities, spanning 4 orders of magnitude, showed that the altitude of breakdown and sprite initiation, as well as their time delays from the lightning discharge are directly related to atmospheric conductivity: higher conductivities produce halo and sprite inception at lower altitudes with longer delays and may hinder sprite formation. We document that variations of 30 times in the lightning current leads to sprite initiation altitudes in the range 66.0–73.5 km, with delays between 1.550 and 34.500 ms, while variations of 4 orders of magnitude in the conductivity profile lead to initiation altitudes 61.0–70.6 km, with delays in the range 3.825–9.825 ms. Consequently, we suggest that lightning characteristics dominate over atmospheric parameters in determining sprites’ initiation altitude and delay. The simulation of a −CG, with a constant current of 30 kA, did not produce a sprite seed, confirming an asymmetry in the response of the atmosphere to positive and negative lightning. This is due to the free electron drift direction that is away from the screening ionization wave, preventing the formation of the streamer seed for the great majority of −CGs. The same does not apply to halos, which depend on the occurrence of breakdown and can be produced by discharges of both polarities.     

Unusually strong magnetic fields in Titan’s ionosphere: T42 case study

Observations of unusually large magnetic fields in the ionosphere indicate periods of maximum stress on Titan’s ionosphere and potentially of the strongest loss rates of ionospheric plasma. During Titan flyby T42, the observed magnetic field attained a maximum value of 37 nT between an altitude of 1200 and 1600 km, about 20 nT stronger than on any other Titan pass and close to five times greater in magnetic pressure. The strong fields occurred near the corotation-flow terminator rather than at the sub-flow point, suggesting that the flow which magnetized the ionosphere was from a direction far from corotation and possibly towards Saturn. Extrapolation of solar wind plasma conditions from Earth to Saturn using the University of Michigan MHD code predicts an enhanced solar wind dynamic pressure at Saturn close to this time. Cassini’s earlier exits from Saturn’s magnetosphere support this prediction because the Cassini Plasma Spectrometer instrument saw a magnetopause crossing three hours before the strong field observation. Thus it appears that Titan’s ionosphere was magnetized when the enhanced solar wind dynamic pressure compressed the Saturnian magnetosphere, and perhaps the magnetosheath magnetic field, against Titan. The solar wind pressure then decreased, leaving a strong fossil field in the ionosphere. When observed, this strong magnetic flux tube had begun to twist, further enhancing its strength.     

An overview of radar soundings of the martian ionosphere from the Mars Express spacecraft

The Mars Express spacecraft carries a low-frequency radar called MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) that is designed to study the subsurface and ionosphere of Mars. In this paper, we give an overview of the ionospheric sounding results after approximately one year of operation in orbit around Mars. Several types of ionospheric echoes are commonly observed. These include vertical echoes caused by specular reflection from the horizontally stratified ionosphere; echoes from a second layer in the topside ionosphere, possibly associated with O⁺ ions; oblique echoes from upward bulges in the ionosphere; and a variety of other echoes that are poorly understood. The vertical echoes provide electron density profiles that are in reasonable agreement with the Chapman photo-equilibrium model of planetary ionospheres. On the dayside of Mars the maximum electron density is approximately 2 × 10⁵ cm⁻³. On the nightside the echoes are often very diffuse and highly irregular, with maximum electron densities less than 10⁴ cm⁻³. Surface reflections are sometimes observed in the same frequency range as the diffuse echoes, suggesting that small isolated holes exist in the nightside ionosphere, possibly similar to those that occur on the nightside of Venus. The oblique echoes arise from upward bulges in the ionosphere in regions where the crustal magnetic field of Mars is strong and nearly vertical. The bulges tend to be elongated in the horizontal direction and located in regions between oppositely directed arch-like structures in the crustal magnetic field. The nearly vertical magnetic field lines in the region between the arches are thought to connect into the solar wind, thereby allowing solar wind electrons to heat the lower levels of the ionosphere, with an attendant increase in the scale height and electron density.     

Earthquake induced dynamics at the ionosphere in presence of magnetic storm

The modifications induced in the dynamics of the ionosphere by the major Japan earthquake (EQ) of March 11, 2011 (epicenter at 38.322°N, 142.369°E, M = 8.9) in presence of a magnetic storm are examined by mapping latitudinal variations of F-layer ionization density (NmF2) from 22 stations covering the epicenter zone. The changes forced into the Total Electron Content (TEC) by the major EQ in the magnetic storm ambiance are also examined from the GPS data collected at Guwahati (26° 10′ N, 91° 45’ E), situated in the major fault system of East Asia. The contributions of pre-seismic electric field as well as of magnetic storm time electric field in the observed density variations are brought into the ambit of discussion. The influence of lower atmosphere in shaping TEC features during the study case is highlighted. The effects of solar activity on density variations during such complex ambiances are also addressed.     

Possible DC electric field in the ionosphere related to seismicity

This paper presents the method for calculation of DC electric field in the atmosphere and the ionosphere generated by model distribution of external electric current in the lower atmosphere. Appearance of such current is associated with enhancement of seismic activity that is accompanied by emanation of soil gases into the atmosphere. These gases transfer positive and negative charged aerosols. Atmospheric convection of charged aerosols forms external electric current, which works as a source of conductivity current in the atmosphere–ionosphere electric circuit. It is shown that DC electric field generated in the ionosphere by this current reaches up to 10 mV/m, while the long-term vertical electric field disturbances excited near the Earth surface do not exceed 100 V/m. Such limitation of the near-ground field is caused by the formation of potential barrier for charged particles at the Earth surface in a process of their transport from soil to atmosphere.     

Modeling and observations of the north–south ionospheric asymmetry at low latitudes at long deep solar minimum

Using the physics based model SUPIM and FORMOSAT-3/COSMIC electron density data measured at the long deep solar minimum (2008–2010) we investigate the longitude variations of the north–south asymmetry of the ionosphere at low latitudes (±30° magnetic). The data at around diurnal maximum (12:30–13:30 LT) for magnetically quiet (Ap ? 15) equinoctial conditions (March–April and September–October) are presented for three longitude sectors (a) 60°E–120°E, (b) 60°W–120°W and (c) 15°W–75°W. The sectors (a) and (b) have large displacements of the geomagnetic equator from geographic equator but in opposite hemispheres with small magnetic declination angles; and sector (c) has large declination angle with small displacement of the equators; vertical E × B drift velocities also have differences in the three longitude sectors. SUPIM investigates the importance of the displacement of the equators, magnetic declination angle, and E × B drift on the north–south asymmetry. The data and model qualitatively agree; and indicate that depending on longitudes both the displacement of the equators and declination angle are important in producing the north–south asymmetry though the displacement of the equators seems most effective. This seems to be because it is the displacement of the equators more than the declination angle that produces large north–south difference in the effective magnetic meridional neutral wind velocity, which is the main cause of the ionospheric asymmetry. For the strong control of the neutral wind, east–west electric field has only a small effect on the longitude variation of the ionospheric asymmetry. Though the study is for the long deep solar minimum the conclusions seem valid for all levels of solar activity since the displacement of the equators and declination angle are independent of solar activity.     

On high-altitude electric fields in the auroral downward current region and their coupling to ionospheric electric fields

The downward field-aligned current region plays an active role in magnetosphere–ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. Other phenomena including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from high-altitude Cluster observations with particular emphasis on the characteristics and dynamics of quasi-static electric field structures. These, extending up to altitudes of at least 4–5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at softer boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft, indicates that the formation of electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures are typically decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The electric field coupling further depends on the current–voltage relationship, which is highly non-linear in the downward current region, and still unrevealed, as to its specific form.     

Numerical simulation of equatorial plasma bubbles over Cachimbo: COPEX campaign

The problem of day-to-day variability in onset of equatorial spread F (ESF) is addressed using data from the 2002 COPEX observational campaign in Brazil and numerical modeling. The observational results show that for values of virtual height of the F layer base less than 355 km at around 18:35 LT, and for the prereversal peak enhancement of the vertical plasma drift (Vp) less than 30 m/s, the spread-F (ESF) was absent on four nights over Cachimbo (9.5°S, 54.8°W, dip latitude = −2.1°). In this work we analyze the geophysical conditions for the generation of the irregularities by comparing the nights with and without the ESF. In the comparison a numerical code is used to simulate plasma irregularity development in an extended altitude range from the bottom of the equatorial F   layer. The code uses the flux corrected transport method with Boris–Book’s flux limiter for the spatial integration and a predictor–corrector method for the direct time integration of the continuity equation for O⁺${\mathrm{O}}^{+}$ and the SOR (Successive-Over-Relaxation) method for electric potential equation. The code is tested with different evening eastward electric fields (or vertical drifts Vp < 30 m/s and Vp > 30 m/s) in order to study the influence of the prereversal enhancement in the zonal electric field on plasma bubble formation and development. The code also takes into account the zonal wind, the vertical electric field and the collision frequency of ions with neutrals and the amplitude of initial perturbation. The simulation shows a good agreement with the observational results of the ESF. The results of the code suggest that the instability can grow at the F layer bottomside by the Rayleigh–Taylor mechanism only when the Vp > 30 m/s. In the analyzed cases we have considered the competition of other geophysical parameters in the generation of plasma structures.     

The thermosphere as a sink of magnetospheric energy: A review of recent observations of dynamics

The study of the dynamics and thermodynamics of the earth's upper atmosphere has made significant progress over the past few years owing to the availability of new global-scale data sets from the Dynamics Explorer satellites. The thermospheric wind and temperature fields at high latitude have been observed to depend strongly on forcing processes of magnetospheric origin. A key momentum source is due to the drag effect of ions convecting in response to electric fields mapped down on the ionosphere from magnetospheric boundary regions. Likewise, an important heat source derives from Joule or frictional dissipation due to ion/neutral difference velocities governed, in turn, by magnetospheric forcing. In this paper we discuss the progress made over the last 2–3 years initiated by the new satellite measurements and we review published data on ion and neutral motions in the context of the energy and momentum coupling between the magnetosphere and the ionosphere/neutral upper atmosphere. The observations indicate the existence of a “flywheel effect” which implies direct feedback from the neutral thermosphere to the magnetosphere via the release of energy and momentum previously “stored” in the neutral thermosphere.     

Importance of electric field measurement over low latitudes at stratospheric heights by balloons

The vertical field in the stratosphere around 35 km is predominantly of atmospheric origin whereas the horizontal electric field at these altitude is mainly of ionospheric origin. The electrical coupling between ionosphere and atmosphere is not known for low latitudes. Balloon borne electric field measurements are planned from Hyderabad, India (geographic latitude 17.5° N) to understand this coupling. Measurement of stratospheric electric fields are also important from the point of view of the sun-weather relationship. It si suggested that the balloon borne electric field measurements are important to understand the electrodynamics of the middle atmosphere.     

由电离层电位所建立的大气电状态

本文在已知电离层电位分布下, 解析地计算了大气电位, 电场和电流强度的全球分布.结果表明, 在大气导电率随高度呈指数增加的情况下, 100km高度上的电离层电位, 几乎无衰减地扫到25km以下.大气电场较强的区域主要在20km以下的低层大气区, 其垂直分量比水平分量大4个数量级.而中高层大气电场较弱, 且两分量量级相当.本文还提出了一种考虑地面形状对大气电场影响的解析方法.      

Neutral atmosphere and crustal magnetization as factors determining differences in plasma convection and magnetic fields distribution in the ionospheres of Mars and Venus

The pattern of the magnetic field/plasma convection can be, to some extent, recovered from the magnetic field measurements by employing either theoretical or numerical models. We use the MAG/ER day-time measurements of the magnetic field at the altitudes from 90 to 180 km during the elliptical orbits of MGS. Analysis of the altitude variation of the characteristics of the large-scale magnetic fields, which were measured some distance away from strong crustal magnetic anomalies, is summarized. The low density of the Martian atmosphere together with the crustal magnetization result in critical differences in plasma convection which are followed by remarkable differences of the magnetic field features within the ionosphere of Venus and Mars (even in its northern hemisphere where the crustal magnetization is, on the average, low) and distribution of currents.     

Physical mechanism of the vertical electric field generation over active tectonic faults

The concept of the Global Electric Circuit (GEC) provides an explanation of the existence of a vertical atmospheric electric field and coupling between the ground and ionosphere. Presently, ionospheric physics pays more attention to electric fields and coupling processes in the polar and auroral regions, whereas in other areas the potential difference between the ground and ionosphere usually is not taken into account. Regional processes exist, however, that are able to significantly affect the GEC parameters and through modification of the ionospheric potential to create plasma density irregularities of different scales within the ionosphere. One such source of ionosphere modification is air ionization in the vicinity of active tectonic faults, which takes place due to increased radon emanation. This paper considers the process of local modification of the GEC and corresponding ionospheric variability due to tectonic activity.     

极区顶部电离层离子上行的太阳活动依赖性研究

利用第23太阳活动周DMSP F12,F13和F15卫星数据,分别对南北半球极区顶部电离层离子上行的太阳活动依赖性进行了研究.结果表明,南北半球上行事件对太阳活动的响应特征基本一致,即高（低）太阳活动时,离子上行通量以及上行数密度较大（小）,但是上行速度及上行发生率较低（高）.以南半球高纬为例,计算得到离子上行通量、数密度、速度及发生率在高低太阳活动条件下的比值分别约为2.26,3.35,0.71,0.51.对离子上行太阳活动依赖性的可能原因进行了分析.不同太阳活动水平下,光致电离及高能粒子沉降的差异会导致电离层离子密度的不同,而电离层离子密度的变化会改变离子elax-elax中性大气之间的碰撞频率,这是影响离子上行发生率的一个重要原因.      

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

Photospheric flows in the active regions (asymmetric and localized Doppler velocities)

We observed 10 active regions through their disk passage during June 25–August 25, 1988, with the Tower Vector Magnetograph (TVM) of Marshall Space Flight Center. The TVM was used in scanning mode to measure the photospheric Doppler velocities with the Line-Center-Magnetogram (LCM) technique in the spectral line of FeI 5250.2 Å. In this paper we present the result of a subset of observations obtained while the active regions were situated away from the solar limb. A wide range of magnetic complexity and associated chromospheric activity characterized these active regions. It was found that the value of zero-crossing wavelength of the integrated Stokes-V profile of two opposite magnetic polarities were different, corresponding to Doppler velocities ranging from ∼100 m s⁻¹ to ∼1475 m s⁻¹. The measurements of relative velocities between different locations, connected by magnetic flux tubes as inferred from YOHKOH soft X-ray and TRACE 171 Å Fe IX images, showed widely different values of dominant localized flows. The region of parasite polarity, which showed recurrent chromospheric activity, was blue shifted with respect to the main “magnetic element” of the same polarity. Some of them were also the sites of sheared magnetic field configuration. The magnitude of the relative velocity between the leading and following polarity is more for the active regions of higher “field asymmetry”.     

Improved precision of virtual height measurements with coherent radio pulse sounding based on the maximum likelihood method

The paper describes a new technique that improves precision of the virtual height measurements by a coherent pulse sounding of the ionosphere. Proposed technique is based on the method of maximum likelihood that matches expected and observed spectral domain signatures of the signal intermixed with the noise. Computer simulations show that our technique allows measurements of the echo virtual height with ∼100 m precision even at a much coarser step of the height sampling in the sounder. In experiment, we expect an average 300 m precision of the virtual height measurements for single echoes received during periods of little spread due to ionospheric irregularities.     

Model for the VLF/LF radio signal anomalies formation associated with earthquakes

The influence of quasi-static electric field of seismic origin on the characteristics of the internal gravity waves (IGWs) in the Earth’s ionosphere is considered. The electric field in the ionosphere arises due to the injection of charged aerosols into the atmosphere, formation of an EMF in the near Earth atmosphere and perturbation of the conductive electric current in the global electric circuit. Amplification of the electric current in seismic zone is accompanied by the formation of perturbation of the lower ionosphere that affects the amplitude and phase of VLF/LF signals. The action of the electric field on the IGWs is connected with the appearance of the Ampere’s force in the ionosphere. In the spectral range of these waves the latter acts on the neutral component of the ionosphere plasma. As the result of this interaction the ionosphere starts to support the discrete spectrum of oscillations. Periods of their maximums increase as numbers of natural sequence. The existence of such peculiarities of the waves in the ionosphere is confirmed by observations.