Geomagnetic Meridian Research Articles

A Unique Case of Complex Interaction Between MSTIDs and Mid‐Latitude Field‐Aligned Plasma Depletions Over Geomagnetic Low‐Mid Latitude Transition Region

AbstractOn a geomagnetically quiet night of July 07, 2018, the allsky imager at Hanle, Leh Ladakh (32.7°N, 78.9°E; Mlat. ∼24.1°N), India captured a number of plasma structures in O (1D) 630.0 nm airglow emission. In a short observational period of around 3 h, two different medium scale traveling ionospheric disturbances and two different mid‐latitude plasma depletions were observed. The most interesting aspect of the night was the drastic difference in the behavior of the two plasma depletions, which simultaneously existed within the imager's field of view for a long time. One of the plasma depletions, which we have termed as mid‐latitude field‐aligned plasma depletion, was nearly field aligned, drifted westward very slowly and became more prominent later in the night. The other depletion showed a much more ambiguous behavior with large reduction in its drift as well as significant change in its orientation relative to the geomagnetic meridian. In the present study, we have tried to explain this contrasting behavior in two different scenarios and have discussed arguments supporting each scenario along with their limitations. These observations are particularly useful considering the location of Hanle, which lies at a transition of geomagnetically low‐mid latitudes, which are governed by different electrodynamics.

Interhemispheric Conjugacy of Concurrent Onset and Poleward Traveling Geomagnetic Responses for Throat Aurora Observed Under Quiet Solar Wind Conditions

AbstractThroat auroras frequently observed near local noon have been confirmed to correspond to magnetopause indentations, but the generation mechanisms for these indentations and the detailed properties of throat aurora are both not fully understood. Using all‐sky camera and magnetometer observations, we reported some new observational features of throat aurora as follows. (1) Throat auroras can occur under stable solar wind conditions and cause clear geomagnetic responses. (2) These geomagnetic responses can be simultaneously observed at conjugate geomagnetic meridian chains in the Northern and Southern Hemispheres. (3) The initial geomagnetic responses of throat aurora show concurrent onsets that were observed at all stations along the meridians. (4) Immediately after the concurrent onsets, poleward moving signatures and micropositive bays were observed in the X components at higher‐ and lower‐latitude stations, respectively. We argue that these observations provide evidence for throat aurora being generated by low‐latitude magnetopause reconnection. We suggest that the concurrent onsets reflect the instantaneous responses of the reconnection signal arriving at the ionosphere, the followed poleward moving signatures reflect the antisunward dragging of the footprint of newly opened field lines, and the micropositive bays may result from a pair of field‐aligned currents generated during the reconnection. This study may shed new light on the geomagnetic transients observed at cusp latitude near magnetic local noon.

SCINDA-GPS derived TEC depletions and amplitude scintillations over Kisumu, Kenya during selected quiet and storm days of 2013 and 2014

Total Electron Content (TEC) depletion and amplitude scintillation (S4) can be derived from, SCINDA-GPS receivers situated in various parts of the equatorial region. In this paper we present results of characterization of TEC depletions and amplitude scintillations over Kisumu, Kenya (Geomagnetic coordinates: 9.64o S, 108.59o E; Geographic coordinates: 0.02o S, 34.6o E) for both selected geomagnetically quiet and geomagnetically disturbed conditions between 1st January 2013 and 31st December 2014 using data derived from the Kisumu NovAtel GSV4004B SCINDA-GPS receiver situated at Maseno University. TEC depletions and amplitude scintillations affect Global Positioning System (GPS) signals in the ionosphere as they propagate from the satellite to the receiver. This study aims to investigate day to day variability of TEC depletions and amplitude scintillations over Kisumu, Kenya during both geomagnetically quiet and geomagnetically disturbed days of 2013 and 2014 which was a high solar activity period for Solar Cycle 24. Seasonal variability of TEC depletions and S4 index is also presented. The Receiver Independent Exchange (RINEX) data for the years 2013 and 2014 was retrieved from the Kisumu SCINDA-GPS receiver, processed to obtain Vertical Total Electron Content (VTEC), S4 and Universal Time (UT) and fed into MATLAB to generate VTEC and S4 plots against UT for each selected quiet and storm day within the 2013 and 2014 period. The obtained results showed a diurnal variation of TEC where TEC was minimum at pre-sunrise, maximum during daytime and minimum during nighttime. The minimum TEC during pre-sunrise and nighttime was attributed to reduced solar intensity while maximum TEC during daytime is attributed to increased solar intensity. Most of the selected quiet and storm days of the years 2013 and 2014 showed TEC depletions and TEC enhancements corresponding with enhanced amplitude scintillations between 1800UT and 20:00UT. This might be attributed to the rapid rise of the F-layer and the increase in the vertical E x B plasma drift due to the Pre-reversal Enhancement (PRE) of the eastward electric field. Post-midnight TEC depletions and amplitude scintillations were observed for some days and this was attributed to the effect of zonal winds which brought post-midnight enhancement of the E x B drift. The percentage occurrence of amplitude scintillations for the selected quiet and storm days exhibited a seasonal dependence with equinoctial months having higher occurrences than the solstitial months. The higher average S4 index during equinoctial months might be attributed to increased solar intensity resulting from the close alignment of the solar terminator and the geomagnetic meridian. Â

Precipitation of Energetic Electrons from the Earth’s Radiation Belt Stimulated by High-Power HF Radio Waves for Modification of the Midlatitude Ionosphere

Based on the results of the experiments performed in 2005–2010 within the framework of the Sura—DEMETER program, we analyze the features of the precipitations of energetic electrons (with energies E ≈ 100 keV) from the Earth’s radiation belt. The modification of the ionospheric F2 region was conducted by means of high-power HF O-mode radio waves radiated in the CW regime. The precipitations were detected using the equipment onboard DEMETER, a French microsatellite. The conditions of precipitation appearance were determined, and it was found that the electron precipitation region was stretched along the geomagnetic meridian to a distance of 1300 km; the size of the region in the transverse direction is about 400 km. It was shown by ionosonde measurements that such precipitations lead to increased absorption of radio waves in the lower ionosphere. It is assumed that the mechanism for precipitation of electrons from the Earth’s radiation belt is determined by the interaction of energetic electrons with VLF radio waves, which are generated due to the interaction of the amplitude-unmodulated O-mode pump wave with the ionospheric plasma near the wave reflection height.

Vertical and Oblique Ionosphere Sounding During the 21 August 2017 Solar Eclipse

AbstractOn 21 August 2017, a total solar eclipse crossed North America. Two Vertical Incident Pulsed Ionospheric Radar ionosondes were operated, one deployed under totality at Lusk, Wyoming, and another south of totality, 313 km away from the geomagnetic meridian in Boulder, Colorado. The two Vertical Incident Pulsed Ionospheric Radar systems were synchronized for precise observation of both vertical and oblique group and phase paths. Hand‐scaled values of virtual heights and critical frequencies are presented. Arrival angles were computed using phase interferometry and used to compute equivalent vertical critical frequencies at the oblique midpoint. Oblique propagation geometry and horizontal gradients allowed measurement of F2 layer during ionosphere G condition where foF1 exceed foF2 after totality.

Interesting Equatorial Plasma Bubbles Observed by All‐Sky Imagers in the Equatorial Region of China

AbstractThis paper reports some interesting equatorial plasma bubbles (EPBs) on 4 and 5 October 2013 and 29 and 30 September 2013 observed by two all‐sky imagers located in the magnetically equatorial region of Hainan (19.5°N; 9.5° geomagnetic latitude) and Guiping (23.8°N; 13.9° geomagnetic latitude), China. The case of 4 and 5 October 2013 shows the following: (1) EPBs had planar wave‐like structures and their wavefronts were parallel to each other before midnight. (2) The angle between the wavefronts of those EPBs and the geomagnetic meridian were ~30°. (3) Near midnight, the higher‐latitude part of one EPB suddenly began to rotate with a large eastward zonal drift, resulting in an unusual C‐shaped EPB that was observed for the first time by an all‐sky imager. (4) After midnight, one EPB merged into another EPB and formed an integrated EPB. The other EPB case on 29 and 30 September 2013 shows the following: (1) The images captured the full evolution processes of an “I” shaped EPB with an angle of ~30° with respect to the geomagnetic meridian. The imagers also observed “S” and “Y” shaped EPBs. (2) The change of EPBs' shape was directly related to the change of EPBs' zonal drift velocities. (3) The angle between the I‐shaped EPBs and the geomagnetic meridian was likely caused by the change of the latitudinal gradient of zonal neutral wind velocities and ionospheric conductivity. (4) The zonal velocity of each branch of the “Y” shape EPB was different, which could be related to the polarization electric fields within each branch.

Dynamic properties of throat aurora revealed by simultaneous ground and satellite observations

AbstractThroat aurora is defined as south‐north aligned auroral arcs equatorward of the dayside cusp aurora and was suggested to be the results of cold magnetospheric plasma interaction with magnetopause reconnection, but its observational properties have not yet been well established. In this paper we carefully examine a sequence of throat auroras observed over Svalbard on 27 December 2003. Observations from 630.0 nm [OI] show that poleward moving throat auroras frequently show brightening followed by dimming in auroral intensity, and sometimes, the brightening throat aurora is a precursor of poleward moving auroral forms. Simultaneous all‐sky images and HF radar backscatter observations along geomagnetic meridian show that the throat aurora brightening is drifting with ionospheric E × B convection and is colocated with enhanced spectral width poleward of the convection reversal boundary (CRB), while its dimming tends to be in the vicinity of the CRB. This leads us to propose that the throat aurora brightening and dimming may be on the open and closed field lines, respectively. Particle data from NOAA 16 confirm that the dimming throat aurora is associated with precipitation of magnetosheath‐like particles mixed with magnetospheric ions (>30 keV), which is characteristic of the low‐latitude boundary layer. For particle data from Defense Meteorological Satellite Program F16, we notice that the dimming throat aurora is associated with lower fluxes of magnetosheath‐like electrons accompanied with magnetospheric electrons, which is most likely on closed field lines. The dynamic properties of throat aurora presented in this paper are thus important to understand its generation mechanisms.

Simultaneous ground‐based optical and HF radar observations of the ionospheric footprint of the open/closed field line boundary along the geomagnetic meridian

AbstractPrevious studies have confirmed that the equatorward boundaries of OI 630.0 nm auroral emissions and broad Doppler spectral widths in Super Dual Auroral Radar Network (SuperDARN) data, the so‐called spectral width boundary (SWB), are good empirical proxies for the dayside open/closed field line boundary (OCB) in the ionosphere. However, both observational techniques are associated with mapping errors. SuperDARN uses a virtual height model for mapping, but it is not well known how the mapping error responds to a changing background ionosphere or transient reconnection events. Optical instruments, such as the meridian‐scanning photometers, have high spatial resolution near zenith, where the mapping error due to the assumed OI 630.0 nm auroral emission height becomes small by comparison. In this work, an adjusted method is introduced to identify the SWB, which does not require temporal smoothing across several scans. The difference in latitude between the SWB, as identified using this method, and the simultaneously observed OI 630.0 nm auroral emission boundary along a common line of sight is compared. Utilizing the OI 630.0 nm boundary as a reference location, we present two case studies observed at different levels of solar activity. In both instances the latitude offset of SWB from the reference location is discussed in relation to the background ionospheric electron density. The compared results indicate that the intake of high‐density solar extreme ultraviolet ionized plasma from subauroral latitudes causes a refraction of the HF radar beam path, which results in an overestimation of range mapping. The adjusted method would thus be a useful tool for identifying the OCB under changing ionospheric conditions in the cusp region.

Correlation-skeleton method for space-time analysis of disturbances in the Pc4-5 period range during magnetic storms

Special method of correlation-skeleton processing of long period MHD disturbed components of the geomagnetic field is developed. To analyze space-time distribution for field components recorded along the meridian 210 at different stages of geomagnetic storms it is applied. This method on the base of post-processing of space-time wavelet analysis of disturbances was performed. The proposal of spectra calculation results in the form of wavelet skeleton pictures is shown. The method allows to find synchronization of oscillatory processes along the geomagnetic meridian and use it as indication of a global magnetic disturbance. Developed algorithm as a diagnostic tool state of the magnetosphere can also be used.

Meridional thermospheric winds over Concepción for the 1979–1989 interval derived from ionosonde observations

The geomagnetic meridian component of thermospheric neutral winds has been determined for the first time using ionosonde observations made at Concepción (36.8°S;73.0°W) over a decade long interval (January 1979–December 1989). Observations correspond to conditions of low geomagnetic activity (Ap⩽20). Winds have been estimated using a servo theory based algorithm including effects of electric fields. Input values are F-region peak heights determined from ionosonde data. Monthly mean diurnal variations were calculated for every month of the interval so as to estimate month-to-month variability. Then seasonal mean diurnal variations were computed and Fourier components of these variations were derived. Seasonal mean winds clearly show a seasonal dependence and the amplitude of the seasonal mean diurnal variation exhibits seasonal and solar cycle dependencies. Fourier analysis shows that harmonic components are significant up to the terdiurnal one in most cases. Servo winds are found to show little resemblance to winds derived using a well-known empirical model. It is concluded that the seasonal mean diurnal variations have common features with those corresponding to three locations on the Antarctic Peninsula, locations in the same longitude sector. Systematic latitude dependences of winds seem to exist when Concepcion and Antarctic Peninsula winds are combined with lower and higher latitude winds derived from FPI observations. Concepcion winds are also generally similar to those for a location in the northern hemisphere, also in the same longitude sector. Somewhat larger differences are found when compared with those for HWM90.

Comparison of the reference mark azimuth determination methods

<p>The knowledge of the azimuth of the reference mark is of crucial importance in the determination of the declination which is defined as the ellipsoidal (geodetic) azimuth of the geomagnetic meridian. The accuracy of the azimuth determination has direct impact on the accuracy of the declination. The orientation of the Declination-Inclination Magnetometer is usually carried out by sighting the reference mark in two telescope faces in order to improve the reliability of the observations and eliminate some instrumental errors. In this paper, different coordinate as well as azimuth determination methods using GNSS (Global Navigation Satellite System) observation techniques within VPPS (High-Precision Positioning Service) and GPPS (Geodetic-Precision Positioning Service) services of the CROPOS (CROatian POsitioning System) system were explained. The azimuth determination by the observation of the Polaris was exposed and it was subsequently compared with the observation of the Sun using hour-angle and zenith-distance method. The procedure of the calculation of the geodetic azimuth from the astronomic azimuth was explained. The azimuth results obtained by different methods were compared and the recommendations on the minimal distance between repeat station and azimuth mark were given. The results shown in this paper were based on the observations taken on the POKU_SV repeat station.</p>

Detection of a large-scale irregularity in the region of the main ionospheric trough according to the data of topside sounding on board the Intercosmos-19 satellite

Complicated ionograms of topside sounding on board the Intercosmos-19 satellite, which were registered on November 26, 1980, in the dusk sector (1800 LT) at the latitudes of the equatorward wall (55°–62° ILAT) of the main ionospheric trough (MIT), are analyzed. They are characterized by the presence of two extra traces at distances larger than the main traces. Approaching the MIT minimum, all traces become more scattered, converge, and join into one strongly diffusive trace. An attempt of interpretation of the complicated ionograms on the basis of trajectory calculations performed by the method of characteristics in the “complex” two-dimensional version (in two mutually intersecting planes) is undertaken. The modeling shows that the extra traces could be related to the presence of a large-scale irregularity stretched along the geomagnetic meridian at the equatorward wall of the MIT. The calculations make it possible to estimate the parameters of the irregularity: the intensity is δfoF2 ∼ 30%, the length is several hundred kilometers, the semi-thickness is 50–60 km, and the height is 350 km. The possible formation causes of the irregularity are discussed. The intensification of the diffuseness of all traces is related to the increase in the intensity of small-scale irregularities, which is usually observed when approaching the MIT minimum.

Studying characteristics of the large-scale ionospheric irregularities induced by high-power HF radio emission with GPS diagnosis

UDC 533.9 We present the results of experimental studies of artificial large-scale irregularities of the ionospheric electron number density with dual-frequency GPS diagnosis. The total electron content was analyzed in the GPS signal trajectory when the satellites passed over the heated region. Spectral composition of the observed variations was determined by wavelet analysis. Characteristic scales of artificial irregularities of the electron number density in the F layer are estimated. It is experimentally proved that the irregularities remain for at least 15–20 min after the transmitter is switched off. A more confident excitation and increased intensity of the irregularities were also confirmed when the beam was inclined south of the magnetic-zenith direction. When the ionosphere is affected by high-power HF radio emission, electron-density irregularities with sizes from tens of centimeters to tens of kilometers are formed in the region of its reflection [1]. Diagnosis of the large-scale (more than 0.5 km) structure of the ionospheric region disturbed by a high-power radio wave was based on raying the ionosphere by decametric radio emission from space radio sources, aspect scattering of HF and UHF radio waves, raying the heated region by UHF signals of low-orbit and geostationary satellites, satellite radio tomography, and electron density in-situ measurements during the flyby of spacecraft and geophysical rockets through and over the heated region (see [1, 2] and the references cited therein). Large-scale irregularities with scales 5–50 km can also be effectively studied using dual-frequency raying by signals of the Navstar (GPS) or GLONASS microwave satellite systems. During propagation through the heated region, such signals acquire an additional phase increment stipulated by the dispersion of radio waves in the ionospheric plasma and linearly related to the total electron content (TEC) on the propagation trajectory [3, 4]. Since the early eighties, this method has been used to determine the TEC in the ionosphere under natural conditions, and since 2007 the Navstar satellites have intensely been used to determine the total electron content in the ionosphere during the experiments on ionosphere modification by high-power HF radio emission. Such studies were based on the HAARP [5] and Sura [2, 6] facilities. A TEC increase correlated with the ionospheric heating was observed in the daytime experiments [2] and [5]. In [6], where the observations were performed mainly after the sunset with the beam of the Sura facility inclined 12 ◦ south in the geomagnetic meridian plane, both a notable TEC increase in the heated region determined by the angular sizes of the antenna pattern and long-period (10–15 min) TEC variations relative to the average value were recorded. On the basis of obtained results, the authors of [6] made a statement about the manifestation of the magnetic-zenith effect as the most effective impact on the ionosphere in the direction along the geomagnetic field.

Afternoon Pc5 geomagnetic pulsations on the Earth’s surface and in the ionosphere (STARE radars)

Two cases when Pc5 geomagnetic pulsations were registered at the IMAGE Scandinavian network of stations and with STARE radars in the afternoon sector (1700–1800 MLT) during the recovery phase of the moderate magnetic storm are analyzed in detail. Using the ground-based observations, it has been indicated that classical quasimonochromatic resonance Pc5 pulsations were observed in the first case (on October 12, 1999; Kp = 5); in this case the maximal amplitude of the spectral maximum at a frequency of 2.5 mHz was registered at Φ ∼ 65°. Two maximums were observed in the spectrum in the second case (on October 13, 1999; Kp = 4): ∼2.5 mHz (the same maximum) and 2.9 mHz; in this case the maximal oscillation amplitude (2.5 mHz) shifted to Φ > 67°. These results were compared with the echo signal intensity simultaneously registered with the STARE Finland radar on a beam oriented along the 105° geomagnetic meridian. The spatial-temporal maps of the Pc5 pulsation amplitude latitudinal distribution (“keograms”), constructed based on the radar measurements in the wide range of geomagnetic latitudes (63°–70°) where the resolution was substantially higher than that of the ground-based observations, made it possible to detect two regions spaced in latitude (Φ ∼ 65° and Φ ∼ 67°–68°) with the simultaneous excitation of oscillations (double resonance?), between which the plasmapause projection was supposedly located.

Development of an ocean surveillance system

Now‐a‐days, underwater sensor networks are gaining considerable importance, for Ocean Surveillance applications. A minimally configurable three‐node sensor network is found to be capable of performing localization as well as tracking of underwater targets. Each node comprises of a surface buoy having processing modules, controller hardware and support electronics for estimating the direction of arrival. The development of the node system comprising of controller and processing modules is presented in this paper. The controller hardware, consisting of a gear assembly, magnetic compass and a precision digital signal controller, helps in sampling the ocean by way of steering the hydrophone arrays and capturing the target emanations, while the processing module performs the computation of the direction of arrivals. The target emissions picked up by the hydrophone arrays are analyzed and processed for computing the direction of signal arrival with reference to the geomagnetic meridian. The direction of maximum...

Nighttime ionosphere–thermosphere coupling observed during an intense geomagnetic storm

The electrodynamics of the ionosphere in the tropical region presents various scientific aspects, which remain subject of intensive investigations and debates by the scientific community. During the year 2002, in a joint project between the Universidade do Vale do Paraíba (UNIVAP) and Universidade Luterana do Brasil (ULBRA), a chain of three Canadian Advanced Digital Ionosondes (CADIs) was established nearly along the geomagnetic meridian direction, for tropical ionospheric studies, such as, changes and response due to geomagnetic disturbances and thermosphere–ionosphere coupling and the generation and dynamics of ionospheric irregularities, in the Brazilian sector. The locations of the three ionosondes stations are São José dos Campos (23.2°S, 45.9°W, dip latitude 17.6°S – under the southern crest of equatorial ionospheric anomaly), Palmas (10.2°S, 48.2°W, dip latitude 5.5°S – near the magnetic equator) and Manaus (2.9°S, 60.0°W, dip latitude 6.4°N – between the geographic and geomagnetic dip equators). It should be pointed out that Palmas and Manaus are located on the opposite sides of the magnetic equator but both are south of the geographic equator. The three CADIs work in time-synchronized mode and obtain ionograms every 5 min. This configuration of the ionospheric sounding stations allowed us to study the F-region dynamics during geomagnetically disturbed period in the meridional direction. Just after the installation and testing of the three CADIs, on September 05, 2002 a coronal mass ejection (CME) left the Sun and about 2 days after the CME left the Sun, it reached the Earth’s magnetosphere and complex and multi step events took place during the period September 07–09. In the study we note that the equatorial stations located north (Manaus, dip latitude 6.4°N) and south (Palmas, dip latitude 5.5°S) of the dip equator presented significant F-layer height asymmetries during the storm main phase. In addition, the low-latitude station SJC (dip latitude 17.6°S) presented decrease in the F-layer densities (negative phase), whereas Palmas presented increase in the F-layer densities (positive phase) during the main phase. This was followed by positive phase at both the stations. During the first night of the recovery phase a strong formation and evolution of large-scale ionospheric irregularities (equatorial spread-F (ESF)) was observed, but on the second night of the recovery phase, there was strong and almost simultaneous sporadic E (Es) formation at all three stations. During the presence of Es, spread-F formation is not observed, indicating the suppression of spread-F, possibly by sporadic E.

Features of stable diffuse arcs observed by means of auroral tomography

Abstract. In this paper we study the spatial distribution of optical volume emission rates and peculiarities of the luminosity intensity within weak diffuse auroral arcs recovered by means of auroral tomography. The tomographic images are obtained from sets of scanning multi-channel photometer data obtained in February 1999 on the Kola Peninsula in Russia at three sites of a chain extending 226 km along the geomagnetic meridian. The 427.8- and 557.7-nm emissions of a 15-s time resolution observed within one hour during low geomagnetic activity are analyzed. We found that the intensity profile of an individual arc along the geomagnetic meridian has an inverted-V-shape. The luminosity maximum altitude decrease by 4–14 km at about 140 km distance in the south-north direction can be observed during two or more diffuse arcs. The parameters of the precipitating electron flux are obtained from an integral equation, which determines the best relationship between the 427.8-nm intensity height profile and an arbitrary particle energy spectrum. A dimensionless function of the energy dissipation is used as the core in the integral equation. The estimated average energy of electron flux, which generated the isolated diffuse arc, is 1–2 keV higher in the central part of the arc in comparison to values at its borders.

The Evolution Characteristics of Geomagnetic Disturbances During Geomagnetic Storm

AbstractWith the data of the geomagnetic meridian observatories in China, 25 geomagnetic storms in the period of 1997 to 1999 have been analyzed by using three kinds of methods which are the method of Natural Orthogonal Components (NOC), Correlation analysis and Fourier analysis. Three steps have been adopted to decompose the storm‐time variation (Dst) relating to the universal time (UT), the solar daily variation (Sq) and the disturbance‐daily variation (SD) depending on the local solar time (LT). The results indicate that (1) The spatial distribution and the temporal evolution characteristics of equatorial symmetric ring current are reflected clearly by the Dst variation. (2) The amplitude of Sq reaches the maximum or the minimum during most main phases in MZL, BMT and QGZ observatories, reflecting the variation of the geomagnetic field with the movement of the Sq focus on latitude. (3) During the main phase, the amplitude of SD reaches the maximum, decreasing slowly in the recovery phase. (4) Correlation analysis and Fourier analyses are effective techniques to extract the variations of Sq and SD.

Comparison of the measured and modeled electron densities, and electron and ion temperatures in the low-latitude ionosphere during 19-21 March 1988

Abstract. We have presented a comparison between the modeled NmF2 and hmF2, and NmF2 and hmF2 which were observed at the equatorial anomaly crest and close to the geomagnetic equator simultaneously by the Akita, Kokubunji, Yamagawa, Okinawa, Chung-Li, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper atmosphere (MU) radar at Shigaraki (34.85°N, 136.10°E, Japan) during the 19-21 March 1988 geomagnetically quiet time period at moderate solar activity near approximately the same geomagnetic meridian of 201°. A comparison between the electron, Te, and ion, Ti, temperatures measured by the MU radar and those produced by the model of the ionosphere and plasmasphere is presented for 19-21 March 1988. It is shown that there is a large disagreement between the measured and modeled hmF2 from about 07:00 UT to about 11:00 UT if the equatorial ExB drift given by Scherliess and Fejer (1999) is used. The required equatorial upward ExB drift is weaker from 03:14 UT to 11:14 UT than that given by Scherliess and Fejer (1999) for the studied time period. The required modification of the ExB drift weakens the effect of the fountain in NmF2 bringing the modeled and measured hmF2 and NmF2, into reasonable agreement. The depth of the equatorial NmF2 trough in the calculated NmF2 is approximately consistent with the measured depth if the modified equatorial ExB drift is used. It has been found that the north-south asymmetries in the observed NmF2 and hmF2 about the geomagnetic equator are mainly caused by the asymmetry in the neutral wind about the geomagnetic equator. In the Northern Hemisphere, the meridional neutral wind taken from the HWW90 wind model and the NRLMSISE-00 atomic oxygen density are corrected so that the model results agree with the ionospheric sounders and MU radar observations. A theory of the primary mechanisms causing the latitude dependence of the morning and evening peaks in Te is developed. The latitude dependence of the magnitudes of these peaks in Te is interpreted in terms of the corresponding dependence of the electron density. The relative role of the ExB drift and the plasma drift caused by the neutral wind in the formation and the dependence of the magnitudes of the morning and evening electron temperature peaks on the geomagnetic latitude is studied.

ULF Pc5‐6 magnetic activity in the polar cap as observed along a geomagnetic meridian in Antarctica

Latitudinal and diurnal distributions of spectral power and spatial coherency parameters of the geomagnetic variations in the Pc5‐6 (1–6 mHz) frequency range are analyzed using data of magnetometer stations in Antarctica. The available stations give the possibility to form a latitude chain along the geomagnetic meridian 40°E stretching from magnetic latitude 69°S to 86°S. Long‐period ULF activity at polar cap latitudes is characterized by lower amplitudes and wider spectra with lower central frequencies as compared with typical auroral Pc5 pulsations. The meridional distribution of average Pc5‐6 spectral power is nonmonotonic and has a minimum near 80°. In general, the low‐frequency broadband ULF activities in the polar cap and at auroral latitudes seem to be decoupled. This long‐period ULF activity in the polar cap could be an image of wave activity in the tail lobes or the manifestation of turbulent component of the ionospheric convection at very high latitudes, but this requires further investigation.