Geomagnetic Field Lines Research Articles

Initial nighttime ionospheric observations with advanced ionospheric probe onboard FORMOSAT-5

FORMOSAT-5 satellite was launched into a sun-synchronous orbit at 720 km altitude with 98.28° inclination on 25 August 2017. The onboard scientific payload, Advanced Ionospheric Probe (AIP) is capable of measuring topside ionospheric ion density, cross-track flow velocities, ion composition and temperature, and electron temperature. Initial observations of nighttime midlatitude ionospheric density and vertical flow velocity variations at 2230 LT sector during a few quiet magnetic days in December 2017 are studied here. Longitudinal density variations in the equatorward edge of midlatitude ionospheric trough (MIT) region are noticed. Accompanied with this density variation, the vertical flow velocities also behave differently. Although the density difference has been stated due to zonal wind effect related to the declination of the geomagnetic field lines, the vertical flow velocity variation seems to play the opposite role. All these density and vertical flow observations in the northern winter hemisphere can only be explained by the longitudinal differences in the diffusion velocity coming down from the protonsphere (plasmasphere). In addition, the hemispheric asymmetry in the vertical flow velocity can also be explained by the interaction between the topside ionosphere and the protonsphere. The observed vertical flow variations near MIT at different longitudes should present a new potential tool for the study of MIT formation.

Electron Wing‐Like Structures Formed at a Negatively Charged Spacecraft Moving in a Magnetized Plasma

AbstractSpacecraft‐plasma interactions are studied with self‐consistent numerical simulations of magnetized plasmas, where electrons are strongly magnetized whereas ions are weakly magnetized. It is found that for a spacecraft in such a magnetized plasma corresponding to a low Earth orbit, electrons can be reflected from a negatively charged spacecraft and then guided by geomagnetic field lines. The reflected electrons can leave a sharp trail like wings if the spacecraft size is greater than an average electron gyroradius of the environment. Such an electron wing‐like structure is associated with propagating Langmuir waves. This results in nontrivial asymmetric electrostatic potentials close to the spacecraft and even farther than the Debye screening distance. The convective electric field also gives rise to a differential potential of the spacecraft with respect to the plasma, resulting in yet another asymmetry in the plasma dynamics and the potential distribution around the spacecraft. These asymmetries in the plasma dynamics can significantly influence in‐situ measurements of space plasma. The results show a good qualitative agreement with actual measurements by a satellite in the polar regions.

A Ray Tracing Simulation of HF Ionospheric Radar Performance at African Equatorial Latitudes

AbstractHigh‐frequency (HF) coherent radars are widely applied for studying electrodynamic processes in the Earth's upper atmosphere and ionosphere. Super Dual Auroral Radar Network (SuperDARN) is an international network of such radars located at high‐ and middle‐latitude regions in the Northern and Southern Hemispheres. A major criterion of the HF coherent radar technique entails that the HF waves propagate orthogonal to the geomagnetic field lines (aspect angle 1°) in the ionospheric irregularity region from where backscatter originates. In this paper we present an HF ray tracing simulation of the performance of three proposed equatorial HF radar systems for the African equatorial sector. We use a number of realistic average ionospheres deduced from the International Reference Ionosphere (IRI) 2012 model and magnetic field geometries from the International Geomagnetic Reference Field (IGRF) to determine likely propagation paths and orthogonality conditions for such radar systems. The east‐west azimuth ( 25° range) has a significant likelihood of achieving a SuperDARN‐type backscatter in the African equatorial sector, mainly due to the prevailing magnetic field geometry. This analysis provides a feasibility study for developing a SuperDARN‐like radar for studying the equatorial ionosphere over the African longitude sector, for example, in determining the technical radar characteristics such as preferable operating frequencies, antenna boresight orientation, and azimuth coverage and provides a simulation of the expected localization of radar backscatter as a function of radar location and beam direction, as well as the time of day, season, and sunspot number.

Coordinated Ground‐Based and Space‐Based Observations of Equatorial Plasma Bubbles

AbstractThis paper presents coordinated and fortuitous ground‐based and spaceborne observations of equatorial plasma bubbles (EPBs) over the South American area on 24 October 2018, combining the following measurements: Global‐scale Observations of Limb and Disk far ultraviolet emission images, Global Navigation Satellite System total electron content data, Swarm in situ plasma density observations, ionosonde virtual height and drift data, and cloud brightness temperature data. The new observations from the Global‐scale Observations of Limb and Disk/ultraviolet imaging spectrograph taken at geostationary orbit provide a unique opportunity to image the evolution of plasma bubbles near the peak height over a large geographic area from a fixed longitude location. The combined multi‐instrument measurements provide a more integrated and comprehensive way to study the morphological structure, development, and seeding mechanism of EPBs. The main results of this study are as follows: (1) The bubbles developed a westward tilted structure with 10–15° inclination relative to the local geomagnetic field lines, with eastward drift velocity of 80–120 m/s near the magnetic equator that gradually decreased with increasing altitude/latitude. (2) Wave‐like oscillations in the bottomside layer and detrended total electron content were observed, which are probably due to upward propagating atmospheric gravity waves. The wavelength based on the medium‐scale traveling ionospheric disturbance signature was consistent with the interbubble distance of 500–800 km. (3) The atmospheric gravity waves that originated from tropospheric convective zone are likely to play an important role in seeding the development of this equatorial EPBs event.

Further complexities on the pre-reversal vertical drift modeling over the Brazilian region: A comparison between long-term observations and model results

The vertical component of the plasma drift, especially the evening-time pre-reversal drift, constitutes an important aspect of the nighttime electrodynamics of the equatorial ionosphere. Over the years, several studies using measurements and models have been performed to understand the characteristics of this process and its implications for the development of the instabilities leading to the plasma bubble formation and ionospheric scintillation. However, the Brazilian region presents some unique features that bring some difficulties for the vertical drift prognosis, which is required for the scintillation forecasting. These features are mainly related to the geomagnetic field lines topology that presents strong differences when compared to those of other equatorial longitudes. In this work, some of the difficulties for the pre-reversal vertical drift modeling and estimation are discussed; also, a dataset containing long-term observation (2001–2009) is compared with a widely used empirical model. The results show an intrinsic trend of underestimation by the model, which seems to be independent of latitude and seasonality thus suggesting an additional contribution arising from sources other than solely the geomagnetic topology. Also, the results indicate that the deviation can vary in the range of 0–40 m/s and the percentage error enhances with increasing values of pre-reversal vertical drift peak and reduces with increasing F10.7 values, thereby, indicating a clear possibility of meridional winds contribution which is not included in the empirical model used and may account for these differences.

The swimming orientation of multicellular magnetotactic prokaryotes and uncultured magnetotactic cocci in magnetic fields similar to the geomagnetic field reveals differences in magnetotaxis between them.

Magnetotactic bacteria have intracellular chains of magnetic nanoparticles, conferring to their cellular body a magnetic moment that permits the alignment of their swimming trajectories to the geomagnetic field lines. That property is known as magnetotaxis and makes them suitable for the study of bacterial motion. The present paper studies the swimming trajectories of uncultured magnetotactic cocci and of the multicellular magnetotactic prokaryote 'Candidatus Magnetoglobus multicellularis' exposed to magnetic fields lower than 80μT. It was assumed that the trajectories are cylindrical helixes and the axial velocity, the helix radius, the frequency and the orientation of the trajectories relative to the applied magnetic field were determined from the experimental trajectories. The results show the paramagnetic model applies well to magnetotactic cocci but not to 'Ca. M. multicellularis' in the low magnetic field regime analyzed. Magnetotactic cocci orient their trajectories as predicted by classical magnetotaxis but in general 'Ca. M. multicellularis' does not swim following the magnetic field direction, meaning that for it the inversion in the magnetic field direction represents a stimulus but the selection of the swimming direction depends on other cues or even on other mechanisms for magnetic field detection.

Influence of Cosmic Ray Invasions and Aerosol Plasma on Powerful Atmospheric Vortices

The Earth’s atmosphere is affected by various ionizing sources. The maximum ionization of atmospheric particles by cosmic rays corresponds to the altitude of formation of tropospheric clouds. In the high-latitude troposphere for the region of the geomagnetic polar cap, in the winter period, the excitation of local cyclonic structures are observed which are accompanied with ice storms, with invasions into middle and subtropical latitudes. The time of excitation of such cyclones is about a day that is comparable with the time of excitation of tornadoes, which are generated at low latitudes. Localization of polar cyclones is not accidental. The region of the polar cap is connected with geomagnetic field lines extended into the tail of the Earth’s magnetosphere. This area is open for the penetration of cosmic rays. The ionization of aerosols in the stratosphere and the upper troposphere by precipitating particles of cosmic rays enhances the vortex activity of the atmosphere. The important role of the aerosol impurity is manifested in the generation of plasma vortices and in the accumulation of energy and mass in the atmosphere by vortices during condensation of moisture. Due to the cascade character of the ionization process, the influence of cosmic radiation turns out to be non-linear and increases with increasing pollution of the atmosphere. Aperiodic electrostatic perturbations, which play a remarkable role in the genesis of vortices, are stochastically excited in plasma inhomogeneities. During the interaction of plasma vortices and Rossby vortices, a large-scale vortex structure is formed and grows.

On some features characterizing the plasmasphere\u2013magnetosphere\u2013ionosphere system during the geomagnetic storm of 27 May 2017

This paper presents how the magnetosphere–plasmasphere–ionosphere system was affected as a whole during the geomagnetic storm peaking on 27 May 2017. The interplanetary conditions, the magnetospheric response in terms of the magnetopause motion, and the ionospheric current flow pattern were investigated using data, respectively, from the WIND spacecraft, from GOES15, GOES13, THEMIS E, THEMIS D and THEMIS A satellites and from the INTERMAGNET magnetometer array. The main objective of the work is to investigate the plasmaspheric dynamics under disturbed conditions and its possible relation to the ionospheric one; to reach this goal, the equatorial plasma mass densities derived from geomagnetic field line resonance observations at the European quasi-Meridional Magnetometer Array (EMMA) and total electron content values obtained through three GPS receivers close to EMMA were jointly considered. Despite the complexity of physical mechanisms behind them, we found a similarity between the ionospheric and plasmaspheric characteristic recovery times. Specifically, the ionospheric characteristic time turned out to be ~ 1.5 days, ~ 2 days and ~ 3.1 days, respectively, at L ~ 3, L ~ 4 and L ~ 5, while the plasmaspheric one, for similar L values, ranged from ~ 1 day to more than 4 days.

Juxtaposed membranes underpin cellular adhesion and display unilateral cell division of multicellular magnetotactic prokaryotes.

Multicellular magnetotactic prokaryotes (MMPs) exhibit peculiar coordination of swimming along geomagnetic field lines. Approximately 40-80 cells assemble, with a helical geometry or axisymmetry, into spherical or ellipsoidal MMPs respectively. To contribute to a comprehensive understanding of bacterial multicellularity here we took multiple microscopic approaches to study the diversity, assembly, reproduction and motility of ellipsoidal MMPs. Using correlative fluorescence in situ hybridization and scanning electron microscopy analysis, we found an unexpected diversity in populations of ellipsoidal MMPs in the Mediterranean Sea. The high-pressure freezing/freeze substitution fixation technique allowed us to show, for the first time, that cells adhere via juxtaposed membranes and are held together by a rimming lattice. Fluorescence confocal microscopy and ultrathin section images revealed not only the one-layer hollow three-dimensional architecture, but also periphery-core unilateral constriction of constituent cells and unidirectional binary fission of the ellipsoidal MMPs. This finding suggests the evolution toward MMPs multicellularity via the mechanism of incomplete separation of offspring. Remarkably, thousands of flagellar at the periphery surface of cells underpin the coordinated swimming of MMPs in response to mechanical, chemical, magnetic and optical stimuli, including a magnetotactic photokinesis behaviour. Together these results unveil the unique structure and function property of ellipsoidal MMPs.

Impact of the Space Charge Distribution in the Model Source of Quasi-Electrostatic Whistler-Mode Waves on the Effective Length of a Short Receiving Antenna

In our previous paper [1] it was shown, using the theory of antennas in plasmas, that the effective length leff of a receiving dipole antenna can be much larger than its geometric length in case quasielectrostatic chorus emissions propagating close to the resonance cone are received. In order to simplify calculations of leff, it was proposed to use a model (“effective”) source of such emissions because taking into account all of the real source properties (that are determined by the nonlinear processes of interaction between waves and charged particles in a wide region of space) would lead to unreasonably cumbersome calculations. The present paper analyzes how the effective length of the spacecraft-borne receiving antenna depends on the parameters of the space charge distribution in the model source of quasi-electrostatic chorus emissions. It is found that the length leff decreases as a power function (with exponent −1/2) with increasing distance (along the group velocity resonance cone) between the model source and the receiving antenna. This relationship is correct up to the distance at which the effective length becomes of the order of the geometric length. At longer distances, the radiation field loses its resonance nature because of the excitation of an electromagnetic (quasi-longitudinal) wave. It is shown that under conditions of the Earth’s magnetosphere, the approximation we used can remain valid up to distances of the order of the geomagnetic field line length, which confirms the importance of the discussed effect for correct interpretation of the electric wave measurement data in the whistler-mode frequency range. It is also shown that the length leff changes by less than 10% when the characteristic scale of spectrum of the charge distribution along the model source varies as Δ ∼ (0.1–1.0) kobs, where the wave number kobs corresponds to the observed spectral maximum of radiation at a given frequency.

Observation of Synchronization Between Instabilities of the Sporadic E Layer and Geomagnetic Field Line Connected F Region Medium‐Scale Traveling Ionospheric Disturbances

AbstractA frequency‐tunable resonance scattering lidar with high temporal/vertical resolutions (1 min/15 m) observed sporadic calcium ion (Ca+s) layers at ~100 km over Tachikawa (geographical/geomagnetic latitude: 35.7°N/27.1°N), Japan, on 21–22 August 2014. Simultaneously, sporadic E (Es) parameters and medium‐scale traveling ionospheric disturbances (MSTIDs) were observed by an ionosonde and Global Navigation Satellite System receiver network, GEONET, respectively. The maximum densities of the Ca+ and electrons in the Es layer had a strong positive correlation. As observation started ~23:30 LT, the Ca+s layer and the associated Es layer descended at ~2.8 km/hr with density irregularities including Kelvin‐Helmholtz billow‐like structures suggesting the presence of background neutral wind shear and instability. And the total electron content variations showed large amplitude associated with the MSTIDs at an altitude of 300 km in synchronization with the Ca+ column abundance surges at 100 km over Tachikawa; in their respective E and F region locations connected by geomagnetic field line these irregularities are found to vary in phase. At ~02:00 LT, the Ca+s layer stopped descending at ~100 km due to larger ion‐neutral collision frequency in the lower altitudes and resided there quietly until sunrise; both Ca+ column abundance enhancements and the large total electron content variation disappeared as the descent of the Ca+s layer stopped, implicating that the MSTID structure cannot be sustained without the density irregularities of the Es layer. This is the first synchronous observation of the coupling between the Es density irregularities and the MSTIDs in the F region along a common magnetic flux tube.

Identification of novel species of marine magnetotactic bacteria affiliated with Nitrospirae phylum.

Magnetotactic bacteria (MTB) are a group of Gram-negative bacteria characterized by synthesizing magnetosomes and swimming along geomagnetic field lines. Phylogenetically, they belong to different taxonomic lineages including Proteobacteria, Nitrospirae, Omnitrophica, Latescibacteria and Planctomycetes phyla on the phylogenetic tree. To date, six Nitrospirae MTB phylotypes have been identified from freshwater or low-salinity environments and described in the literature. Here, we report the identification of two Nitrospirae MTB phylotypes collected, for the first time, from the marine environment. Both have a spherical morphology with a cell size of ~ 5 μM and similar motility but are different colours (black-brown and ivory-white) under the optic microscope. They synthesized bullet-shaped iron-oxide magnetosomes that were arranged in multiple bundles of chains. Moreover, the cytoplasm of the black-brown Nitrospirae MTB contained sulphur inclusions that conferred on cells a rough, granular appearance. Phylogenetic analysis based on their 16S rRNA gene sequences revealed that they are two novel species and cluster with the previously reported MTB affiliated with the phylum Nitrospirae, thus extending the distribution of Nitrospirae MTB from freshwater to the marine environment.

Do the Chinese Astronomical Records Dated AD 776 January 12/13 Describe an Auroral Display or a Lunar Halo? A Critical Re-examination

The enhancement of carbon-14 in tree rings around AD 774/775 has generated wide interest in solar activity at that time. The historical auroral records have been examined critically. Of particular interest was the “white vapour” observed in China on AD 776 January 12/13. Both Usoskin et al. (Astron. Astrophys.55, L3, 2013; U13) and Stephenson (Adv. Space Res.55, 1537, 2015; S15) interpreted this record as an auroral display. Subsequently, Neuhäuser and Neuhäuser (Astron. Nachr.336, 225, 2015; NN15) proposed five “criteria” for the likeliness of aurorae and on this basis rejected an auroral interpretation. Instead, they interpreted it as a lunar halo, and suggested there were no auroral records as a proxy of solar activity in the interval AD 774 – 785. We consider if their “lunar halo hypothesis” and their auroral criteria could be of use in future researches on historical auroral candidates. We first show a counter-example for the lunar halo hypothesis from a parallel record on 1882 November 17, which was seen as a whitish colour, in the southerly direction, and near the Moon. We then consider NN15’s criteria on colour, direction, and sky brightness and investigate other counter-examples from early-modern auroral observations. We also consider the extension of the white vapour in AD 776 according to the distribution of Chinese asterisms, and show that its large extension was inconsistent with the lunar halo hypothesis. Conversely, the streaks of white vapour penetrating the eight Chinese asterisms can be reproduced if we consider auroral-ray structures at altitudes between 97 km and 170 km, along geomagnetic field lines between the L-shells L=1.55 and 1.64. Our investigations show that we should consider candidate auroral records in historical documents not on the basis of the newly suggested a priori criteria by NN15 but based on all the available observational evidence.

Ionospheric Non-linear Effects Observed During Very-Long-Distance HF Propagation

A new super-long-range wave propagation technique was implemented at different HF heating facilities. The HF waves radiated by a powerful heater were scattered into the ionospheric waveguide by the stimulated field aligned striations. This waveguide is formed in a valley region between the E- and F- layers of the ionosphere. The wave trapping and channeling provide super-long-range propagation of HF heater signals detected at the Ukrainian Antarctic “Academik Vernadsky” Station (UAS) which is many thousand kilometers away from the corresponding HF heating facility. It is objectives of this paper to study excitation of the ionospheric waveguide due to scattering of the HF heating wave by the artificial field aligned irregularities. In addition, the probing of stimulated ionospheric irregularities can be obtained from analyses of the signals received at far distance from the HF heater. The paper uses a novel method of scattering of the HF radiation by the heating facility for diagnostics of nonlinear effects at the super-long radio paths. Experiments were conducted at three different powerful HF facilities: EISCAT (Norway), HAARP (Alaska) and Arecibo (Puerto Rico) and by using different far spaced receiving sites. The key problems for super-long-range propagation regime is feeding of the ionospheric waveguide. Then the energy needs to exit from the waveguide at a specific location to be detected by the surface-based receiver. During our studies the waveguide feeding was provided by scattering of the HF waves by the artificial ionospheric turbulence (AIT) above the HF heater. An interesting opportunity for channeling of the HF signals occurs due to the aspect scattering of radio waves by field aligned irregularities (FAI), when the scattering vector is parallel to the Earth surface. Such FAIs geometry takes place over the Arecibo facility. Here FAI are oriented along the geomagnetic field line inclined by 43 degrees. Since the Arecibo HF beam is vertical, the aspect scattered waves will be oriented almost horizontally toward the South. Such geometry provides unique opportunity to channel the radio wave energy into the ionospheric waveguide and excites the whispering gallery modes.

Resonant ULF Absorption Revealed by Auroral Doppler Radar Data

STARE Doppler data on electron drift velocities detected in the auroral ionosphere for weakly disturbed conditions are analyzed. The Doppler measurements are first averaged along each radar beam. The power spectral density (PSD) as a function of frequency is then calculated by discrete Fourier transforms of averaged signals for every radar beam. Deep stepwise drops (about 10 dB) in spectral powers of the ultra-low-frequency (ULF) range are revealed for all radar beams. These PSD drops are interpreted as manifestations of resonant ULF absorption, which occurs in the eigenfrequency continuum of standing Alfven waves excited on geomagnetic field lines. A variational analysis that models PSD decreases by stepwise profiles of the mean spectral powers is proposed. This analysis provides the least-squares fitting of model profiles to PSD decreases calculated by the data. The frequency of the mean PSD step determined for each radar beam is treated as the minimum frequency of resonant ULF absorption revealed by this beam during the given auroral event. Averaged over all beams, this frequency for the analyzed event is 4.9 ± 0.2 mHz.

Observations of Low‐Latitude Traveling Ionospheric Disturbances by a 630.0‐nm Airglow Imager and the CHAMP Satellite Over Indonesia

AbstractWe report the first comparison of ground and satellite measurements of low‐latitude traveling ionospheric disturbances (TIDs). Three TID events were simultaneously observed by a 630.0‐nm airglow imager and the CHAllenging Minisatellite Payload (CHAMP) satellite at Kototabang, Indonesia (geographic coordinates: 0.2° S, 100.3° E, geomagnetic latitude: 10.6° S). In 630.0‐nm airglow images of all three events, there are clear southward‐moving structures. Events 1 and 2 are a single pulse with horizontal scales of ∼500–1,000 km. Event 3 shows five wave fronts with a horizontal scale size of 500–1,000 km. All three TIDs are medium‐scale TIDs. Horizontal wavelengths of both airglow intensity at an average emission altitude of 250 km and CHAMP neutral density variations measured at 400 km are estimated by fitting a sinusoidal function to the observed data. The estimated horizontal wavelengths for airglow and neutral density data are 1,031 and 880 km for event 1 and 560 and 420 km for event 3, respectively. These values between airglow and CHAMP are comparable, suggesting both instruments are observing the same wave. For event 1, the CHAMP electron density mapped along the geomagnetic field line onto the airglow altitude does not show wave structure similar to the airglow variation. For events 2 and 3, the plasma density did not show wavy structures similar to the waves seen in the airglow image and CHAMP neutral density. These results suggest that the TIDs observed in airglow images are not caused by ionospheric plasma instability but by gravity waves in the thermosphere.

Bioinspired multifunctional layered magnetic hybrid materials

Nature has taken millennia to come up with unique solutions for providing materials with properties tailored toward versatile demands, making use of the very limited resources available in natural environments. Today, these biomaterials can be used as inspiration by combining and ‘remixing’ the concepts that nature displays to create new bioinspired materials. Here, the authors present materials combining the structural and functional elements of multiple biominerals: the inorganic–organic lamellar structure responsible for the high fracture toughness of nacre; highly mineralized composites, which give different mollusk teeth their very high hardness and strength; and the particle orientation and magnetic anisotropy of magnetosomes, giving magnetotactic bacteria a sensitive means to navigate along geomagnetic field lines. The authors show how the mechanical properties of a composite material can be improved with the addition of each of these elements. Small-angle neutron scattering studies and molecular simulation give additional insights into the mineralization from the very first attached ions to the finished composite.

Nanokristalle für die Magnetfeldorientierung – Biogenese von Magnetosomen

Alphaproteobacteria of the genus Magnetospirillum use dedicated organelles, the magnetosomes, to navigate along geomagnetic field lines in their natural environment. Magnetosomes are nanocrystals of magnetite (Fe3O4) which are biomineralized within specific membrane vesicles and become aligned into chains by a dedicated cytoskeleton. This article also highlights the potential for functionalization and application of these bacterial magnetic nanoparticles.

3-D Radar Imaging of E-Region Field-Aligned Plasma Irregularities by Using Multireceiver and Multifrequency Techniques

This paper reports the first use of a 3-D radar imaging for observing E-region field-aligned plasma irregularities (FAIs) in the midlatitude ionosphere. Five carrier frequencies equally spaced between 46.25 and 46.75 MHz were transmitted alternately with consecutive radar pulses, and 20 receivers were employed for receiving the radar echoes. The experiments were carried out using the middle and upper atmosphere radar (34.85°N, 136.10°E), with the radar beams steered to the geographic north and 6.6° north by east, at the zenith angles of 51.0° and 51.5°, respectively. The 2-D imaging with the echoes collected by the 19 of the 20 receivers revealed that in zonal dimension, the off-beam direction of arrival (DOA) of the echo region changed alternately between positive and negative values, which can be attributed to the highly localized FAIs drifting zonally through the radar beam. On the other hand, the off-beam DOA in meridional dimension was negative and positive, respectively, at higher and lower range locations, which were supposed to be due to the meridional drift component of FAIs. A combination of 5 frequencies and 19 receivers achieved a 3-D imaging of the FAI structures, illustrating small-scale FAI striations and bubblelike plasma structures in the radar volume. Moreover, declination of geomagnetic field line was examined from the imaged 3-D brightness distribution in the radar volume; the estimated declination of geomagnetic field line was in agreement with that computed from the International Geomagnetic Reference Field model in 2011. This paper has demonstrated some capabilities of radar imaging for ionospheric investigation.

Modulation of Low‐Altitude Ionospheric Upflow by Linear and Nonlinear Atmospheric Gravity Waves

AbstractThis study examines how thermospheric motions due to gravity waves (GWs) drive ion upflow in the F region, modulating the topside ionosphere in a way that can contribute to ion outflow. We present incoherent scatter radar data from Sondrestrom, from 31 May 2003 which showed upflow/downflow motions, having a downward phase progression, in the field‐aligned velocity, indicating forcing by a thermospheric GW. The GW‐upflow coupling dynamics are investigated through the use of a coupled atmosphere‐ionosphere model to examine potential impacts on topside ionospheric upflow. Specifically, a sequence of simulations with varying wave amplitude is conducted to determine responses to a range of transient forcing reminiscent of the incoherent scatter radar data. Nonlinear wave effects, resulting from increases in amplitude of the modeled GW, are shown to critically impact the ionospheric response. GW breaking deposits energy into smaller scale wave modes, drives periods of large field‐aligned ion velocities, while also modulating ion densities. Complementary momentum transfer increases the mean flow and, through ion‐neutral drag, can increase ion densities above 300 km. Ionospheric collision frequency (cooling) and photoionization effects (heating), both dependent on ionospheric density, modify the electron temperature; these changes conduct quickly up geomagnetic field lines driving ion upflow at altitudes well above initial disturbances. This flow alters ion populations available for high‐altitude acceleration processes that may lead to outflow into the magnetosphere. We have included a representative source of transverse wave heating which, when supplemented by our GWs, illustrates strengthened upward fluxes in the topside ionosphere.