Time and Altitude spread F echoes distribution over the Christmas Island VHF radar

The goal of this work is to study the time and altitude echoes characteristics under different solar and seasonality conditions using the VHF radar RTI images. The occurrence of equatorial spread F depends on the existence of conditions that can seed the Raileight-Taylor instability, and these conditions can change with solar flux, seasonality, longitude distributions, and day-to-day variability. So, the equatorial spread F is observed as its time and altitude occurrence. The VHF radar of Christmas Island (2.0° N, 157.4° W, 2.9° N dip latitude) has been operational in the equatorial region for some time now, allowing long-term observations. The occurrence of echoes during solar minimum conditions are observed all throughout the night since the post reversal westward electric field is weaker than the solar maximum and the possibilities for the vertical plasma drift to become positive are larger. On other hand, echoes during solar maximum will be controlled by dynamics near the time of the Pre-reversal Peak (PRE). Our results indicate peak time occurrence of echoes along this period shows a well-defined pattern, with echoes being distributed as closer to local sunset during solar maximum and around/closer midnight during solar minimum conditions, meanwhile, the peak altitude occurrence of echoes shows a slightly regular pattern with higher altitude occurrences during solar maxima and lower altitudes during solar minimum conditions.

Westward Drift of Ionospheric Plasma Irregularities over a Low to Mid-latitude Transition Region in Indian Sector

We report the observation of plasma depletions/plumes in the F region ionosphere over a low to middle latitude transition region in the Indian sector. The observation of these plasma depletions is based on the data obtained in May 2019 through the all-sky airglow CCD imager installed in the campus of University of Kashmir, Srinagar (34.12 °N, 74.83 °E, magnetic latitude 25.91 °N). The depletions on the two consecutive nights of 05 and 06 May 2019 are aligned along the North-South (N-S) direction and drift westward. Several depletion bands along with some enhancement bands are seen in the 630-nm airglow images throughout the two nights. The observed structures show certain characteristics similar to Medium Scale Traveling Ionospheric Disturbances (MSTIDs) but these airglow features are not completely periodic. Further, in the observed depletion bands some East-West asymmetries are observed along with the structured tree-like branches of the airglow depletions. Some depletion bands even bifurcate leading to the inference that the structures are signatures of plasma irregularities rather than the usual MSTIDs observed in low-mid latitude transition region. The westward drift of the depletions especially during geomagnetic quiet times over this region makes this study significant since it offers a possible evidence that shows extension of spread F irregularities from the mid latitude region to the low-mid latitude transition region. In this paper, we point out some possible mechanisms related to the occurrence of plasma depletions at this region and their westward movement during geomagnetic quiet times.

Spread-F characteristics over Tucumán near the southern anomaly crest in South America during the descending phase of solar cycle 24

Ionospheric F-region irregularities can acutely affect navigation and communication systems. To develop predictive capabilities on their occurrence, it is key to understand their variabilities in a wide range of time scales. Previous studies at low latitudes in South America have been performed mostly in the eastern region. However, there are still few reports on the spread-F over Argentina owing to a lack of ionosonde data. This work presents the analysis of the spread-F (range spread-F and frequency spread-F) and plasma bubble occurrence characteristics near the southern crest of the Equatorial Ionization Anomaly in Argentina (Tucumán, 26.8°S, 65.2°W; magnetic latitude 15.5°S). We used ionosonde and Global Positioning System (GPS) data from November 2014 to December 2019 for different solar and geomagnetic conditions. The data show that spread-F and plasma bubble occurrence rates peak in local summer and are minimum in equinox and winter, respectively. There is a negative correlation between each type of spread-F and solar activity, whereas the opposite happens for plasma bubbles. Geomagnetic activity suppresses the generation of spread-F in equinox and summer and enhances it in winter. Plasma bubble occurrence is higher during disturbed days than during quiet days, but under medium solar activity, summer months register more plasma bubbles in quiet conditions. Range spread-F observed in winter under low solar activity is not associated with plasma bubbles originated at the magnetic equator. These results contribute to the knowledge necessary to improve the prediction of the spatial and temporal distribution of the night-time ionospheric irregularities.

Climatology of Spread F over Tucumán from Massive Statistical Analysis of Autoscaled Data

Automatic ionogram interpretation methods developed for real-time ionospheric monitoring can be applied in retrospective studies to analyze large quantities of data. The Autoscala software, implemented for such a purpose, includes a routine for automatic detection of diffused echoes known as spread F, which appear in ionograms due to the presence of ionospheric irregularities along the radio signal path. The main objective of this routine is to reject bad quality ionograms. This new capability was used in a climatological study including a large number of ionograms recorded at the low-latitude ionospheric station of Tucumán (26.9° S, 294.6° E, magnetic latitude 15.5° S, Argentina). The study took into account different levels of geomagnetic and solar activity from 2012 to 2020. The results demonstrate the capability of Autoscala to capture the main signature characteristics of spread F and the temporal evolution of the ionosphere peak heigh hmF2, capturing the post-sunset plasma surge that precedes development of spread F. Maximum occurrence of spread F is observed in local summer, with a tendency to shift before midnight with increasing solar activity. Other new climatological details that emerged from the study are illustrated and briefly discussed, dealing with connection with geomagnetic activity, and morning hmF2 behavior after extremely marked nighttime spread F occurrence.