Digital Ionosonde Measurements Research Articles

Pre-sunrise uplift and sunrise downward excursion in the F-region vertical plasma drift: Observations from the mid-latitude station Nicosia

The characteristics of mid-latitude vertical plasma drift, with a focus on pre-sunrise hours for Nicosia station, Cyprus, based on digital ionosonde measurements, have been investigated and are reported for the first time. The pre-sunrise vertical plasma drift is defined by an uplift prior to sunrise, followed by a downward drift, which is referred to as sunrise downward excursion (SDE). The pre-sunrise uplift at mid-latitudes seems to be apparent due to ‘thinning’ of the F- layer bottom side. The observed uplift is followed by a sudden descend which is also ‘apparent’ and is attributed to the downward movement of the ionisation peak during sunrise. The pre-sunrise uplift is more significant in winter but the downward drift is observed irrespective of the season.

Evaluation of IRI predicted characteristics of ionospheric F1 layer by ionosonde observations in Nicosia, Cyprus

This paper presents an investigation of F1 layer characteristics derived from manually scaled digital ionosonde measurements at the low-middle latitude European station in Nicosia, Cyprus (geographical coordinates: 35°N, 33°E, geomagnetic lat. 29.38°N, I=51.7°) from low to high solar activity conditions (2009–2014) and their comparison with IRI-2012 predictions. It assesses the predictability of occurrence probability by employing all three options included in IRI-2012: IRI-95, Scotto-97 no L, and Scotto-97 with L. Results show that Scotto-97 no L option slightly overestimates the occurrence probability but predicts better than IRI-95 option, whereas IRI-95 option closely reflects the time range for F1 layer appearance. The Scotto-97 with L option slightly underestimates the occurrence probability of foF1+ledge type. A seasonal variation marked with higher occurrence probability in the equinoctial months than in winter is observed, both at low and high solar activity. IRI predictions match reasonably well observed values of foF1 with maximum 0.3MHz over/under estimation at low solar activity. At high solar activity, IRI slightly overestimates the values of foF1 throughout the year with a varying degree from 0.3 to 1.3MHz, except for the months of August and September, when the values are underestimated by 0.3MHz. A seasonal variation with higher foF1 values during equinoctial months than that of summer and winter is noticed in the observations at high solar activity in 2014. Long-term solar variation in the observed foF1 values were evident. Deviation of IRI-2012 predictions from observed foF1 values increases at high solar activity. Observed values of hmF1 demonstrate no distinct diurnal variation. However, these are lower in winter than those of summer and equinoctial months. No significant long-term solar variation in hmF1 was detected during the entire period under consideration. Throughout 2009–2014, with a few exceptions, IRI generally underestimates hmF1 during January-April but overestimates during September-December. Over/underestimation goes as high as 23km.

Comparison of peak characteristics of the F2 ionospheric layer obtained from the Cyprus Digisonde and IRI-2012 model during low and high solar activity period

We investigate first the climatology expressed by diurnal and seasonal variations of the critical frequency (foF2) and the peak height (hmF2) of the F2-layer derived from digital ionosonde measurements at the low-middle latitude European station in Nicosia, Cyprus (geographical coordinates: 35°N, 33°E, geomagnetic lat. 29.38°N, I=51.7°). Monthly median hourly values of the F2-layer peak characteristics are obtained using manually scaled data during the 5-year period 2009–2013. The observational results are then compared with the International Reference Ionospheric Model (IRI-2012) predictions using both URSI and CCIR coefficients. It is shown that the semi-annual pattern of daytime foF2 characterized by higher values at equinoxes than either solstices as well as the winter anomaly phenomenon demonstrate strong solar activity dependence. An annual pattern of night-time foF2 is also detected with lower values in winter and higher in summer. The seasonal variation of daytime hmF2 is evident and peaks of hmF2 at pre-sunrise and post-sunset hours are identified during December. The IRI-2012 model is capable to capture the main diurnal and seasonal patterns of foF2 and hmF2. The highest overestimation of daytime foF2 is noted at equinoxes and solstices except from March, October, December of 2011, and June of 2013. Significant foF2 underestimation is observed at evening and after midnight during February and March of 2009. Large positive discrepancies between the modeled and observed hmF2 values are noticed during the deep solar minimum year 2009. Overall, IRI-model estimates are more accurate for hmF2 than foF2 over Cyprus and for the examined period.

Local ionospheric electron density profile reconstruction in real time from simultaneous ground-based GNSS and ionosonde measurements

The purpose of the LIEDR (local ionospheric electron density profile reconstruction) system is to acquire and process data from simultaneous ground-based total electron content (TEC) and digital ionosonde measurements, and subsequently to deduce the vertical electron density distribution above the ionosonde’s location. LIEDR is primarily designed to operate in real time for service applications and, for research applications and further development of the system, in a post-processing mode. The system is suitable for use at sites where collocated TEC and digital ionosonde measurements are available. Developments, implementations, and some preliminary results are presented and discussed in view of possible applications.

Bottomside profile shape parameters during low solar activity and comparisons with IRI-2007 model

Modern digital ionosonde measurements at low–middle latitude station, New Delhi, India, are used to assess the IRI-2007 model for the bottomside profile shape parameters B0 and B1 during solar minimum. Comparative analysis shows that in general, the IRI (B0 Table) option reveals better agreement with the B0 observations during daytime in all the seasons, while outside this time period, the IRI (Gulyaeva) predicted B0 values are closer to the observations. For B1 parameter, both the options in the IRI reproduce similar diurnal variations in all the seasons and are closer to observed values except during pre-sunrise and post-sunset hours.

Comparison between IRI predictions and digital ionosonde measurements of hmF2 at New Delhi during low and moderate solar activity

This paper deals with the diurnal and seasonal variations of height of the peak electron density of the F2-layer (hmF2) derived from digital ionosonde measurements at a low–middle-latitude station, New Delhi (28.6°N, 77.2°E, dip 42.4°N). Diurnal and seasonal variations of hmF2 are examined and comparisons of the observations are made with the predictions of the International Reference Ionosphere (IRI-2001) model. Our study shows that during both the moderate and low solar activity periods, the diurnal pattern of median hmF2 reveals a more or less similar trend during all the seasons with pre-sunrise and daytime peaks during winter and equinox except during summer, where the pre-sunrise peak is absent. Comparison of observed median hmF2 values with the IRI during moderate and low solar activity periods, in general, reveals an IRI overestimation in hmF2 during all the seasons for local times from about 06 LT till midnight hours except during summer for low solar activity, while outside this time period, the observed hmF2 values are close to the IRI predictions. The hmF2 representation in the IRI model does not reproduce pre-sunrise peaks occurring at about 05 LT during winter and equinox as seen in the observations during both the solar activity periods. The noontime observed median hmF2 values increase by about 10–25% from low (2004–2005) to high solar activity (2001–2002) during winter and equinox, while the IRI in the same time period and seasons shows an increase of about 10–20%. During summer, however, the observed noontime median hmF2 values show a little increase with the solar activity, as compared to the IRI with an increase of about 12%.

Sunward polar cap convection

This is a study of the conditions for which there is sunward convection in the central polar cap. The study of convection direction uses digital ionosonde measurements from two central polar cap stations (Resolute Bay, corrected geomagnetic (CG) latitude 83.3°N, and Eureka, CG latitude 88.67°N) and presents statistical results about pure sunward convection events. The sunward convection events in the central cap region are observed only when the interplanetary magnetic field (IMF) Bz is >2 nT. For weak northward IMF (<3 nT), sunward convection events are confined to the region near the noon cusp. For strong northward IMF conditions (>3 nT), sunward convection occurs at all local times, indicating the expansion of reverse convection to most of the central polar cap. There are very notable IMF By effects on the pattern of sunward convection. For IMF By positive the axis of the pattern of reverse convection appears to rotate toward the postnoon, whereas for IMF By negative the axis is rotated toward the prenoon. These axis orientations are compatible with the expected locations of lobe cell merging. Another significant IMF By effect is that the duration of sunward convection intervals when IMF By is negative is about the same as the duration of the northward IMF intervals, whereas for positive IMF By the sunward convection intervals usually last only for a short duration (∼1 hour). However, if the northward IMF interval is much longer than 1 hour, there is a sequence of these ∼l‐hour sunward convection events. The convection pattern during northward IMF appears to be compatible with a four‐cell form.

Signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above Casey, Antarctica

Signatures of the ionospheric cusp in HF digital ionosonde measurements of plasma drift made at the polar cap station Casey, Antarctica (−80.8° geomagnetic latitude), are investigated. Measurements recorded during the campaign interval February 13–17, 1996, are considered in this case study because the summer dipole tilt effect, and an interplanetary magnetic field (IMF) northward condition on February 16, were favorable for the detection of the cusp at a higher than usual latitude. On February 14 and 15 the magnitude of the IMF was about 4–6 nT, and the station probably passed just poleward of the cusp. The most general signatures of the cusp were enhanced electric field and electric field turbulence shown by increased drift velocity and velocity scatter in the convection throat, respectively. Broadband, unstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were associated with the intervals of enhanced convection turbulence. A major cusp event occurred above Casey on February 16, when the magnitude of the IMF increased from about 5 to > 10 nT. Cusp signatures during this event included the drift velocity surging to large values just before and after an interval during which the F region echoes were lost because of an absence of F region ionization and the formation of electron density patches. The loss of echoes was only partly explained by increased absorption and scatter of the transmitted radio waves. Although the spectral width of Doppler peaks increased, this, by itself, was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near‐vertical interferometry. However, signatures of the cusp were easily recognized in digisonde data, and the cusp's location and dynamics can be monitored using digisondes.

Digital ionosonde measurements of the height variation of drift velocity in the southern polar cap ionosphere: Initial results

During the late austral summer of 1995–1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° corrected geomagnetic (CGM) latitude), in an experimental drift mode with the aim of resolving the height variation of drift velocity in the polar cap ionosphere. We devised control programs for a Digisonde Portable Sounder 4 to collect data at separate frequency‐range gates corresponding to the E and F regions to investigate the differences in their motions. During a 4‐day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic field (V⊥) were 85 m s−1 in the E region and 485 m s−1 in the F region (using 10 m s−1 bins and echoes from all heights in each region). Vertical profiles of drift velocity were obtained by sorting echoes into 10‐km group‐height bins. For measurements obtained within ±3 hours of magnetic noon the average profile showed that in the lower E region V⊥ increased approximately exponentially with true height. The corresponding velocity scale height was <9.0 km at 105 km, where the gradient was >46.7 m s−1 km−1. The mean value of V⊥ leveled off to about 700 m s−1 above 120 km, where it remained up to the F region peak height. The vertical gradient was caused by the increase in collision frequencies at the lower heights. The F region field‐aligned component of drift (V‖) showed a strong diurnal variation, with mean values of −30 m s−1 near noon and +60 m s−1 during the night at a height of 180 km. The average over the whole day reveals a net upward drift of 30 m s−1. This behavior is attributed to the interaction between the meridional components of the generally antisunward neutral wind (UN) and perpendicular drift (V⊥s) moving plasma down the field lines during the day and up the field lines during the night, with UN and V⊥s having net equatorward values when averaged over all day. While the E region drift direction tended to be aligned with the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in direction, suggesting a smaller‐scale size and lifetime for the E region structures giving rise to the echoes. There were events lasting over 2 hours during which the drifts in the two regions were clearly resolved into different azimuths (by nearly 180° for two events). These transient directional shears show the time variability in the phase transition between an F region collisionless, magnetized plasma driven by the E × B/B2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.

F layer ionization patches in the polar cap

Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

Polar cap F‐layer auroras

Polar cap auroras were measured at Thule Air Base, Greenland in December 1979, using an all sky imaging photometer. These images combined with digital ionosonde measurements, show the orientation, structure, and drift motion of sun‐aligned, F layer auroras. Drifts in both the dawn‐to‐dusk and dusk‐to‐dawn direction were observed. These auroras were not visible on the all sky camera films; however, coincident Defense Meteorological Satellite Program (DMSP) precipitating electron measurements show that these arcs are produced by fluxes of low energy (E<500 ev) electrons. Simultaneous satellite‐to‐ground signal amplitude measurements show that these auroras are accompanied by km scale size ionospheric irregularities, which cause strong amplitude fluctuations (scintillation) on the received satellite signal.