Topside Ionosphere

The Arecibo incoherent scatter radar facility was operated on February 26, 1998, and was used to observe the total solar eclipse that occurred over the Caribbean. A maximum of 87% obscuration was observed over Arecibo at 1430 LT (1830 UT). The radar was operated using an experimental technique, which uses a 300 μs single/multi-frequency pulse, to gather data from the altitude range 146–2412 km. The Sheffield University plasmasphere ionosphere model was used to interpret the measurements. The electron temperature was found to have decreased by 600 K at 400 km altitude, but the magnitude of the decrease becomes smaller with increasing altitude. This is shown to be the result of the lesser degree of obscuration of the solar disk at latitudes north of Arecibo. Conjugate point photoelectron heating effects are also shown to play a significant role in the electron energy balance during the eclipse. The H+ ion temperature exhibited a response to the eclipse, with temperatures being around 200 K lower than expected at the time of maximum obscuration. There was relatively little variation observed in the O+ temperature. The response of the topside ionosphere is characterized by a downward motion arising from the contraction of the plasma due to reduced plasma temperatures. This is most clearly seen in the O+-H+ transition altitude which falls by 200 km. The transition altitude fully recovers within 2 hours after the eclipse. The location of the transition altitude acts to mitigate the effects of the eclipse on the topside electron densities.

We have used observations from both the Bennett ionmass spectrometer and the retarding potential analyzer on board the Atmosphere Explorer E satellite to study the longitudinally averaged O+, H+, and He+ concentrations from 150 to 1100 km in the equatorial ionosphere during the 1975-1976 solar minimum. Our results suggest that the ion mass spectrometer measurements need to be increased by a factor of 2.15 to agree with the densities from the retarding potential analyzer and with ground-based measurements. The peak H+ concentrations are about 2.5 × 104 cm-3 during the day and 104 cm-3 at night and vary little with season. The O+/H+ transition altitude lies between 750 and 825 km during the day and between 550 and 600 km at night. He+ is a minor species at all altitudes; its concentration is highly variable with a maximum value of about 103 cm-3 during equinox daytime.

We describe recent developments in and results from topside ncoherent scatter radar (ISR) measurements at Arecibo, PR, emphasizing helium ion measurements. Recent improvements in the data taking modes and the data processing permit isolation of the concentrations of oxygen, helium and hydrogen ions between the F region peak and 2000 km with about 10 minute time resolution. The need for the three ion non‐linear least squares fits is justified by use of the goodness of fit; a two ion fit is shown to be unacceptable. The new measurements are optimized for the rapid height variations in the solar minimum nighttime ionosphere by replacing the traditional 1 ms pulse with a 500 µs pulse. We show results from one day of a five day experiment during the Spring of 1994. For these equinox solar minimum conditions, the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude (ht), forming a distinct layer, most noticeable during the night. The maximum helium ion concentrations tend to be quite low, 2 or 3 × 10³ cm−3, or 10–20% of the topside plasma at the peak of the He+ layer. The transition altitude, ht, varies from about 1200 km to 1400 km during the day to near 500 km at night, and the He+ layer follows this altitude variation. Finally we show that the location of the layer near ht and its intensification during the night can be explained using ambipolar diffusion equations.

We describe a new technique to derive neutral atomic oxygen density, [O], in the upper thermosphere using coincident incoherent scatter radar (ISR) and airglow emission observations from Arecibo Observatory. The technique exploits the nearly resonant charge exchange coupling between neutral and ionized hydrogen and oxygen that serves as the dominant chemical source and sink of protons near and above the F region peak. Under charge exchange production and loss of H+, the proton continuity equation can be solved for [O] using twilight H density profiles derived from measured H emission brightness at 656.3 nm together with ion density, temperature, and flux obtained simultaneously by the Arecibo ISR. We present both equilibrium and nonequilibrium solutions for [O] between 500 and 1500 km during a single quiescent nighttime interval under moderate solar activity. Comparisons with theoretical expectations and with MSIS-model calculations of O density are used to identify the altitude and local time extent over which the technique is justified. These comparisons generally support technique validity between 600 and 800 km, where sufficient reactant densities are present to validate the charge exchange formulation of the continuity equation. Equilibrium solutions for [O] near 650–700 km exhibit excellent agreement with MSIS estimates before midnight, but deviations arising from ion transport become increasingly significant both above this height and as dawn approaches. Incorporation of measured proton flux gradients into the nonequilibrium solutions improves agreement between the derived and modeled estimates significantly after midnight, while the minor nonequilibrium contributions during several hours before midnight lend additional support for the presence of charge exchange equilibrium.

This work presents, for the first time, the analysis of the occurrence of ionospheric irregularities during geomagnetic storms at Tucumán, Argentina, a low latitude station in the Southern American longitudinal sector (26.9°S, 294.6°E; magnetic latitude 15.5°S) near the southern crest of the equatorial ionization anomaly (EIA). Three geomagnetic storms occurred on May 27, 2017 (a month of low occurrence rates of spread-F), October 12, 2016 (a month of transition from low to high occurrence rates of spread-F) and November 7, 2017 (a month of high occurrence rates of spread-F) are analyzed using Global Positioning System (GPS) receivers and ionosondes. The rate of change of total electron content (TEC) Index (ROTI), GPS Ionospheric L-band scintillation, the virtual height of the F-layer bottom side (h'F) and the critical frequency of the F2 layer (foF2) are considered. Furthermore, each ionogram is manually examined for the presence of spread-F signatures.

The results show that, for the three events studied, geomagnetic activity creates favorable conditions for the initiation of ionospheric irregularities, manifested by ionogram spread-F and TEC fluctuation. Post-midnight irregularities may have occurred due to the presence of eastward disturbance dynamo electric fields (DDEF). For the May storm, an eastward over-shielding prompt penetration electric field, (PPEF) is also acting. A possibility is that the PPEF is added to the DDEF and produces the uplifting of the F region that helps trigger the irregularities. Finally, during October and November, strong GPS L band scintillation is observed associated with strong range spread-F (SSF), that is, irregularities extending from the bottom-side to the topside of the F region.

The purpose of this paper is to evaluate the performance of the TSMP-assisted Digisonde (TaD) topside profiling technique. We present systematic comparisons between electron density profiles and TEC parameters extracted from TaD model with (a) CHAMP-derived TEC parameters, (b) CHAMP reconstructed profiles, (c) ground based GPSderived TEC parameters, and (d) profiles reconstructed from RPI/IMAGE plasmagrams. In all cases, TaD follows the general trend of plasmaspheric observations derived from the above datasets. Especially during storm cases, TaD shows remarkable agreement with the variations of the ground based GPS-derived TEC parameters. Overall, the comparison results shows that TaD method can be adopted by EURIPOS to provide the electron density distribution up to plasmaspheric heights in real-time.

The purpose of this paper is to evaluate the performance of the TSMP-assisted Digisonde (TaD) topside profiling technique. We present systematic comparisons between electron density profiles and TEC parameters extracted from TaD model with (a) CHAMP-derived TEC parameters, (b) CHAMP reconstructed profiles, (c) ground based GPSderived TEC parameters, and (d) profiles reconstructed from RPI/IMAGE plasmagrams. In all cases, TaD follows the general trend of plasmaspheric observations derived from the above datasets. Especially during storm cases, TaD shows remarkable agreement with the variations of the ground based GPS-derived TEC parameters. Overall, the comparison results shows that TaD method can be adopted by EURIPOS to provide the electron density distribution up to plasmaspheric heights in real-time.

The incoherent scatter radar mode and data extraction method used in topside experiments at the Arecibo Observatory are discussed. Helium ion concentrations in the lower topside ionosphere over Arecibo are presented for low and high solar flux periods of the same season (October). Data from 8–9 October 1988, 21–22 October 1995, 26–27 October 1997, and 12–13 October 2001 are compared. The data from October 1988 and 2001 are representative of high solar flux conditions, while the other two periods (1995 and 1997) are from low solar flux conditions. All data sets represent geomagnetic quiet periods over Arecibo and serve as examples of typical equinox topside conditions. For solar minimum the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude (inline equation) during the night, usually around 550–650 km. The He+ number densities tend to be quite low, 2 or 3 × 10^3 cm−3 or <15% of the topside plasma density at the peak of the layer. For the higher solar flux case, however, He+ forms a more distinct layer at an altitude that is slightly above inline equation at night. The O+-to-He+ transition altitude (inline equation) is reported to be below inline equation after the postsunset collapse in these solar-maximum conditions, meaning that He+ is the dominant species. We find that the relative abundance of He+ can reach 60 to 65% of the topside plasma at the peak of this layer during these equinox conditions or up to 4 × 104 cm−3, with inline equation as low as 700 km and the He+ to H+ transition (inline equation) extending over 1000 km at night.

We have measured both proton (T H+ ) and oxygen ion (T O+ ) temperatures simultaneously from spectra obtained in the lower topside ionosphere using the Arecibo incoherent scatter radar (ISR). This is the first time that two ion temperatures and three ion compositions have been measured from the same IS spectrum, and, as far as we know, one of the first measurements of both ion temperatures in the ionosphere. First we describe our spectral‐domain analysis technique, emphasizing the ability to determine goodness of fit, which allows of the need for and effect of including a parameter to be numerically evaluated. We then show that common T H+ and T O+ allow insufficient freedom, and that separate temperatures are sufficient. A small negative bias in [He+] parameter is removed when the second temperature is allowed to be free while the H+ fraction is not significantly affected. We also show that the measurement cannot be made at altitudes where little O+ is present. The difference between T H+ and T O+ is zero (or at least very small) during the night, and rises to several hundred degrees at about 500 km shortly after sunrise. During the middle of the day the temperature difference declines (presumably due to higher densities) and then rises again before sunset.

Ionospheric irregularities can severely degrade radio communication and navigation systems. Geomagnetic storms may affect the generation of these irregularities in a way that is not yet fully understood. To improve the forecasting of this phenomenon, we need to study the ionosphere in different regions of the world, and in particular in the equatorial ionization anomaly (EIA) where irregularities are usually more intense. This study analyses the effect of geomagnetic storms on ionospheric irregularities. We examined the occurrence of irregularities at the southern crest of the EIA in Argentina (Tucumán, 26.9°S, 294.6°E, dip latitude 15.5°S) during three intense and one moderate geomagnetic storm of different solar sources, between 2015 and 2018. We used data from an ionosonde, a Global Positioning System receiver and magnetometers. Ionogram spread-F, the F-layer bottom side (h'F), the critical frequency of the F2-layer (foF2), the rate of TEC index and the S4 scintillation index were analysed. The data show irregularities were present as range spread-F and moderate TEC fluctuations in one storm: 27 May 2017 (a coronal mass ejection CME-driven storm occurred on local winter), and were absent in the other events. We suggest that eastward disturbance dynamo electric field and over-shielding prompt penetration electric fields may create favourable conditions for developing these irregularities, whereas westward storm time electric fields might inhibit the growth of irregularities during the other storms considered. During co-rotating interaction region CIR-driven storms, the westward disturbance dynamo electric field may be associated with the non-occurrence of irregularities.