P. T. Jayachandran

Equatorial F region zonal plasma irregularity drifts under magnetospheric disturbances

Equatorial F region plasma drift velocities measured by a digital ionosonde (CADI) that was recently installed in Fortaleza, Brazil, are used to investigate magnetospheric disturbance effects in the vertical (zonal) and zonal (vertical) velocities (electric fields). For the first time we report evidence of large fluctuations in irregularity zonal drift velocities (∼50–180 m/s) associated with magnetospheric disturbances. The fluctuations in the zonal velocity, anti correlated with those in vertical velocity, are unlikely to be produced by prompt penetration of disturbance meridional electric field of high latitude/magnetospheric origin. A mechanism is proposed to explain the velocity fluctuations that involves: (1) Hall polarization vertical electric field in the E-layer that is field line mapped on to F-layer, and (2) electric field caused by vertical current arising from divergence in field line integrated zonal Pedersen current; both produced by the primary disturbance zonal electric field. Enhanced nighttime E region conductivity with possible spatial gradients, a requirement for the functioning of this mechanism, is observed to be present from other simultaneous measurements, whose source is suggested to be particle induced ionization in the south Atlantic Magnetic Anomaly (SAMA) zone, as known also from previous studies.

HF detection of slow long-lived E region plasma structures

During the equinox and winter seasons, and in the range 300–1000 km the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar often detects extended patches of coherent echoes with remarkably uniform properties and low Doppler speeds, in the range 0 to 200 m/s. Typically, these echoes last for ∼3 hours, and are observed between 1300 and 2300 MLT, at times of moderate to high Kp values. The echo Doppler shift changes systematically with azimuthal angle and a vector reconstruction of the implied drift indicates westward velocities in the range 150 to 250 m/s, well below the threshold speed associated with Farley-Buneman waves. When ionosonde observations are available, they invariably show the presence of a thick sporadic E layer. This feature, plus the facts that the IMF By is always negative and that the echoes are equatorward of the regions of discrete precipitation (as indicated by comparison with coincident DMSP satellite observations), indicate that the echoes are associated with the diffuse aurora in regions where the electric field is of the order of 10 mV/m or less. We infer from these echo properties that the irregularities are triggered by a primary gradient-drift mechanism which then cascades to the observed structures through weakly turbulent mode-coupling processes. Several events were observed during special multifrequency experiments using the Saskatoon SuperDARN radar. It was found that the Doppler speed, power, and spectral width all increase systematically with increasing radar frequency. The findings for Doppler speed and power appear to arise, at least in part, from the increase in height of the radar echoes with increasing frequency. The frequency dependence of spectral width may be related to instability lifetimes; it was found to agree well with the results of numerical simulations [Keskinen et al., 1979].

Presunrise spread F at Fortaleza

At Fortaleza, Brazil, in the equatorial zone about 400 km south of the magnetic equator a presunrise (secondary) maximum of spread F occurrence is observed during sunspot minimum and, in particular, during December solstice. The spread F takes the form of patches of irregularities that are convecting eastwards at ∼50 m s−1. Most of the patches are collocated with bottomside bulges of the ionosphere. Our measurements indicate that these bottomside bulges are unstable due to a gradient-drift instability that is slowly growing and produces the spread F. The bulges themselves seem to be evidence of a Rayleigh-Taylor instability process.

Polar cap Sporadic-E: part 2, modeling

Abstract A model of polar cap Sporadic-E is presented which incorporates several novel features, including the results of recent laboratory studies on metallic ion recombination. The initial Sporadic-E is assumed to be produced by gravity waves interacting with metallic and ambient NO + and O 2 + ions. This produces transient ionization enhancements in the E region. In the winter night, these enhancements undergo dissociative recombination with electrons and disappear, but in summer reionization of the metallic atoms by charge exchange with ambient E region ions can maintain Sporadic-E as thin intense layers at an altitude of about 100 km for many hours.

Determining receiver biases in GPS-derived total electron content in the auroral oval and polar cap region using ionosonde measurements

Global Positioning System (GPS) total electron content (TEC) measurements, although highly precise, are often rendered inaccurate due to satellite and receiver differential code biases (DCBs). Calculated satellite DCB values are now available from a variety of sources, but receiver DCBs generally remain an undertaking of receiver operators and processing centers. A procedure for removing these receiver DCBs from GPS-derived ionospheric TEC at high latitudes, using Canadian Advanced Digital Ionosonde (CADI) measurements, is presented. Here, we will test the applicability of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated GPS vertical TEC (vTEC) measurements to corresponding International GNSS Service IONEX-interpolated vTEC map data. We demonstrate that the bias values determined using the CADI method are largely independent of the topside model (exponential, Epstein, and α-Chapman) used. We further confirm our results via comparing bias-calibrated GPS vTEC with those derived from incoherent scatter radar (ISR) measurements. These CADI method results are found to be within 1.0 TEC units (TECU) of ISR measurements. The numerical methods tested demonstrate agreement varying from within 1.6 TECU to in excess of 6.0 TECU when compared to ISR measurements.

High-latitude ionospheric response to co-rotating interaction region- and coronal mass ejection-driven geomagnetic storms revealed by GPS tomography and ionosondes

Positive ionospheric anomalies induced in the polar cap region by co-rotating interaction region (CIR)- and coronal mass ejection (CME)-driven geomagnetic storms are analysed using four-dimensional tomographic reconstructions of the ionospheric plasma density based on measurements of the total electron content along ray paths of GPS signals. The results of GPS tomography are compared with ground-based observations of F region plasma density by digital ionosondes located in the Canadian Arctic. It is demonstrated that CIR- and CME-driven storms can produce large-scale polar cap anomalies of similar morphology in the form of the tongue of ionization (TOI) that appears on the poleward edge of the mid-latitude dayside storm-enhanced densities in positive ionospheric storms. The CIR-driven event of 14–16 October 2002 was able to produce ionospheric anomalies (TOI) comparable to those produced by the CME-driven storms of greater Dst magnitude. From the comparison of tomographic reconstructions and ionosonde data with solar wind measurements, it appears that the formation of large-scale polar cap anomalies is controlled by the orientation of the interplanetary magnetic field (IMF) with the TOI forming during the periods of extended southward IMF under conditions of high solar wind velocity.

Ionospheric irregularities: The role of the equatorial ionization anomaly

The availability of similar ionosondes at four locations in India covering the regions from the equator to the northern crest of the equatorial ionization anomaly (EIA) has provided a unique opportunity to study the role of the EIA and related processes in the occurrence of spread F. The study is conducted during equinoctial months of high solar activity period (Rz=145), when the probability of occurrence of spread F is maximum. This study also deals with vertical movement of the F layer due to both electric field and neutral winds and the occurrence of spread F. The present study shows that a well-developed anomaly beyond the latitude of 18°N is one of the base-level conditions for the generation of spread F. The total integrated vertical movement is the controlling factor for the onset of spread F and not the peak value of h′F alone. There seems to be no threshold value in the altitude of the F layer (h′F) for the occurrence of spread F at all stations. The direction and magnitude of meridional winds that are crucial in the generation of spread. F showed higher values of poleward wind on non-spread-F days, indicating that the irregularity growth is suppressed due to the poleward wind.

A comparison between large-scale irregularities and scintillations in the polar ionosphere

This work in China is supported by the National Basic Research Program (grant 2012CB825603), the National Natural Science Foundation (grants 41274149, 41274148, and 41574138), the Shandong Provincial Natural Science Foundation (grant JQ201412), and the young top-notch talent of “Ten Thousand Talent Program.” J. Moen is supported by the Research Council of Norway grant 230996. The work at Reading University was supported by STFC consolidated grant ST/M000885/1. SuperDARN is a collection of radars funded by national scientific funding agencies in Australia, Canada, China, France, Japan, South Africa, United Kingdom, and United States of America. M. Lester acknowledges support from STFC grant ST/K001000/1 and NERC grant NE/K011766/1. The GPS TEC acquisition effort is led by A. J. Coster at MIT Haystack Observatory. We thank the MIT Haystack Observatory for generating GPS TEC data and making them available through the Madrigal Database (http://madrigal.haystack.mit.edu/) and the University of New Brunswick for running CHAIN and providing scintillation data through database (http://chain.physics.unb.ca/chain/pages/gps/). We also acknowledge the NASA OMNIWeb for IMF and solar wind data (http://omniweb.gsfc.nasa.gov/html/sc_merge_data1.html), WDC C1, Kyoto for the AE/AL and SYM-H indices (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html), and the Johns Hopkins University Applied Physics Laboratory (JHU/APL) for providing online OVATION (http://sd-www.jhuapl.edu/Aurora/ovation/ovation_display.html).

The nature of GPS differential receiver bias variability: An examination in the polar cap region

While modern GPS receiver differential code bias estimation techniques have become highly refined, they still demonstrate unphysical behavior, namely, notable solar cycle variability. This study investigates the nature of these seasonal and solar cycle bias variabilities in the polar cap region using single-station bias estimation methods. It is shown that the minimization of standard deviation bias estimation technique is linearly dependent on the users choice of shell height, where the sensitivity of this dependence varies significantly from 1 total electron content unit (1 TECU = 1016 el m−2) per 4000 km in solar minimum winter to in excess of 1 TECU per 90 km during solar maximum summer. Using an ionosonde, we find appreciable shell height variability resulting in bias variabilities of up to 2 TECU. Comparing northward face Resolute Incoherent Scatter Radar (RISR-N) measurements to a collocated GPS station, we find that RISR-derived GPS receiver biases vary seasonally but not with solar cycle. RMS differences between bias estimation methods and observation between 2009 and 2013 were found to range from 2.7 TECU to 3.4 TECU, depending on method. To account for the erroneous solar cycle variability of standard bias estimation approaches, we linearly fit these biases to sunspot number, removing the trend. RMS errors after sunspot detrending these biases are reduced to 1.91 TECU. Also, these ISR-derived and sunspot-detrended biases are fit to ambient temperature, where a significant correlation is found. By using these temperature-fitted biases we further reduce RMS errors to 1.66 TECU. These results can be taken as further evidence of temperature-dependent dispersion in the GPS cabling and antenna hardware.

Electron density and electric field over Resolute Bay and F region ionospheric echo detection with the Rankin Inlet and Inuvik SuperDARN radars

Joint observations of the Rankin Inlet and Inuvik Super Dual Auroral Radar Network HF radars and Resolute Bay (RB) Canadian Advanced Digital Ionosonde are used to assess the electron density at the F region peak and the electric field magnitude as factors affecting echo detection over RB. We demonstrate that the radars show similar diurnal and seasonal variations in ionospheric echo occurrence. During nighttime and at radar frequencies of ~12 MHz, optimum densities for both radars are shown to be ∼ 1.4 × 105 cm− 3, ~1.8 × 105 cm− 3, and ~2.0 × 105 cm− 3 for winter, equinox, and summer, respectively. During daytime, optimum densities are larger by (0.2 − 0.3) × 105 cm− 3. Observations at lower radar frequencies of ~10 MHz show smaller required densities during nighttime, by ~0.3 × 105 cm− 3. Optimum electric fields for the moments of echo detection over RB are found to be 5–25 mV/m with no clear threshold effect and any seasonal dependence. The presented data suggest that for echo detection, favorable propagation conditions along the entire path of radio waves toward the scattering volume are important.