Carlos R. Martinis

Brightening of 630.0 nm equatorial spread‐F airglow depletions

[1] Observations from the Boston University all-sky imaging system at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N mag), show an unusual behavior of nighttime 630.0-nm airglow depletions. Associated with equatorial spread-F (ESF), these structures move eastward before reversing their motion and become airglow enhancements. Few other cases have been found, all during December solstices. For the case study presented here, data from the Arecibo incoherent scatter radar and the Republic of China Scientific Satellite (ROCSAT-1) provide supporting information. The radar shows that around local midnight the background zonal and meridional plasma motions reverse to westward and southward, respectively. ROCSAT-1 shows enhanced ion density, i.e., a low-latitude plasma blob, above the bright feature recorded by the all-sky imager, indicating a possible connection between both phenomena. Drifts parallel to the magnetic field are observed only in the region where the enhancement occurs. One possible interpretation of this change in the brightness of the depleted structure involves the influence of northward meridional winds and a reversal in the zonal drift motion, most likely caused by a zonal wind reversal.

Zonal neutral winds at equatorial and low latitudes

Abstract Fabry–Perot interferometric (FPI) measurements of thermospheric zonal neutral winds at Arequipa, Peru ( 16.7° S , 71.5° W , −2.7° dip ), and Carmen Alto, Chile ( 23.1° S , 69.4° W , −10.2° dip ), were collected during the solar minimum periods of September–October 1996 and 1997. The data set included 39 nights from Arequipa and 14 nights for Carmen Alto, with 8 nights of simultaneous observations. Analysis of averaged results found the peak evening zonal neutral wind speed of ∼127±15 m / s eastward for the Arequipa observatory, which is located near the magnetic equator, to occur between 21:30 and 22:30 LT. In contrast, the peak evening zonal winds of ∼100±10 m / s eastward observed from Carmen Alto, which is located near the crest of the equatorial ionization anomaly (EIA), occurred ∼0.5– 1 h later. These measurements represent the first case of groundbased FPI observations of the so called equatorial temperature and wind anomaly (ETWA) over such a small latitude range in the same longitude sector. This reduction in speed of ∼20–25% at Carmen Alto relative to Arequipa is attributed to increased ion drag at Carmen Alto caused by the higher electron density within the EIA region at altitudes of 220– 300 km . Model studies were conducted using electron density and neutral atmosphere parameters form the parameterized ionospheric model (PIM) and the mass spectrometer incoherent scatter (MSIS) models, respectively, to calculate the ratio of ion–neutral collision frequencies at the two sites. We found that the increase in electron density within the EIA was sufficient to account for the observed reduction in the zonal wind. Thus, this analysis confirms the dominant role of ion drag in modulating thermospheric dynamics at equatorial latitudes. A comparison of the FPI results with the predictions by two current neutral wind models, the Horizontal Wind Model-90 and the NCAR Thermospheric Ionosphere Electrodynamics General Circulation Model (TIEGCM), reveals that neither is able to reproduce accurately the latitude dependence reported here. Model refinements for electrodynamics and improved resolution are suggested.

Storm time response of the midlatitude thermosphere: Observations from a network of Fabry‐Perot interferometers

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

New radar observations of temporal and spatial dynamics of the midnight temperature maximum at low latitude and midlatitude

Presented here are several cases of midnight temperature maximum (MTM) observations using the Millstone Hill incoherent scatter radar (ISR) and Arecibo ISR. The MTM, a temperature enhancement in the upper atmosphere (at ~300 km altitude), is a poorly understood phenomenon as observations are sparse. An upward propagating terdiurnal tide and coupling between atmospheric regions may play a large part in the generation of the MTM, yet this phenomenon and its implications are not fully understood. Two nights (6 March 1989 and 12 July 1988) show clear cases of the MTM occurring between 30 and 34°N with amplitudes of ~100 K and at ~18°N with amplitudes of ~40 K. The MTMs occurred later at the higher latitude. Experiments in 2013 also show a clear MTM at 34° and 36°N from 250 to 350 km altitude. The ionospheric measurements presented here demonstrate a new application of a well-established technique to study atmospheric parameters and allow us to study the latitudinal extent of the MTM. The results provide evidence of the phenomenon occurring at latitudes and altitudes not previously sampled by radar techniques, showing that the MTM is not just an equatorial process, but one that can easily reach midlatitudes. Simultaneous measurements with a Fabry-Perot interferometer allow us to compare the neutral temperatures with the ion temperature. Overall, these are key observations that point to large-scale effects that can help constrain model outputs at different heights and latitudes.

A dayside plasma depletion observed at midlatitudes during quiet geomagnetic conditions

In this study we investigate a dayside, midlatitude plasma depletion (DMLPD) encountered on 22 May 2014 by the Swarm and GRACE satellites, as well as ground-based instruments. The DMLPD was observed near Puerto Rico by Swarm near 10 LT under quiet geomagnetic conditions at altitudes of 475–520 km and magnetic latitudes of ∼25°–30°. The DMLPD was also revealed in total electron content observations by the Saint Croix station and by the GRACE satellites (430 km) near 16 LT and near the same geographic location. The unique Swarm constellation enables the horizontal tilt of the DMLPD to be measured (35° clockwise from the geomagnetic east-west direction). Ground-based airglow images at Arecibo showed no evidence for plasma density depletions during the night prior to this dayside event. The C/NOFS equatorial satellite showed evidence for very modest plasma density depletions that had rotated into the morningside from nightside. However, the equatorial depletions do not appear related to the DMLPD, for which the magnetic apex height is about 2500 km. The origins of the DMLPD are unknown, but may be related to gravity waves.

The night when the auroral and equatorial ionospheres converged

An all-sky imaging system at the McDonald Observatory (30.67°N, 104.02°W, 40° magnetic latitude) showed dramatic ionospheric effects during a moderate geomagnetic storm on 1 June 2013. The auroral zone expanded, leading to the observation of a stable auroral red (SAR) arc. Airglow depletions associated with equatorial spread F (ESF) were also seen for the first time at such high magnetic latitude. Total electron content measurements from a Global Positioning System (GPS) receiver exhibited ionospheric irregularities typically associated with ESF. We explore why this moderate geomagnetic disturbance leads to such dramatic ionospheric perturbations at midlatitudes. A corotating interaction region-like driver and a highly contracted plasmasphere caused the SAR arc to occur at L shell ~ 2.3. For ESF at L ~ 2.1, timing of the storm intensification, alignment of the sunset terminator with the central magnetic meridian, and sudden variations in the westward auroral electrojet all combined to trigger equatorial irregularities that reached altitudes of ~ 7000 km. The SAR arc and ESF signatures at the ionospheric foot points of inner magnetosphere L shells (L ~ 2) represent a dramatic convergence of pole to equator/equator to pole coupling at midlatitudes.

AMISR‐14: Observations of equatorial spread F

A new, 14-panel Advanced Modular Incoherent Scatter Radar (AMISR-14) system was recently deployed at the Jicamarca Radio Observatory. We present results of the first coherent backscatter radar observations of equatorial spread F(ESF) irregularities made with the system. Colocation with the 50 MHz Jicamarca Unattended Long-term studies of the Ionosphere and Atmosphere (JULIA) radar allowed unique simultaneous observations of meter and submeter irregularities. Observations from both systems produced similar Range-Time-Intensity maps during bottom-type and bottomside ESF events. We were also able to use the electronic beam steering capability of AMISR-14 to “image” scattering structures in the magnetic equatorial plane and track their appearance, evolution, and decay with a much larger field of view than previously possible at Jicamarca. The results suggest zonal variations in the instability conditions leading to irregularities and demonstrate the dynamic behavior of F region scattering structures as they evolve and drift across the radar beams.

C/NOFS Remote Sensing of Ionospheric Reflectance

Alfvn waves play critical roles in the electrodynamic coupling of plasmas at magnetically conjugate regions in near-Earth space. Associated electric (E*) and magnetic (dec B*) field perturbations sampled by sensors on satellites in low-Earth orbits are generally super positions of incident and reflected waves. However, lack of knowledge about ionospheric reflection coefficients (alpha) hinders understanding of generator outputs and load absorption of Alfvn wave energies. Here we demonstrate a new method for estimating using satellite measurements of ambient E* and dec B* then apply it to a case in which the Communication Navigation Outage Forecasting System (CNOFS) satellite flew conjugate to the field of view of a 630.0 nm all-sky imager at El Leoncito, Argentina, while medium-scale traveling ionosphere disturbances were detected in its field of view. In regions of relatively large amplitudes of E* and B*,calculated values ranged between 0.67 and 0.88. This implies that due to impedance mismatches, the generator ionosphere puts out significantly more electromagnetic energy than the load can absorb. Our analysis also uncovered caveats concerning the methods range of applicability in regions of low E* and B*. The method can be validated in future satellite-based auroral studies where energetic particle precipitation fluxes can be used to make independent estimates of alpha.

Concurrent observations at the magnetic equator of small-scale irregularities and large-scale depletions associated with equatorial spread F

In 2014 an all-sky imager (ASI) and an Advanced Modular Incoherent Scatter Radar consisting of 14 panels (AMISR-14) system were installed at the Jicamarca Radio Observatory. The ASI measures airglow depletions associated with large-scale equatorial spread F irregularities (10–500 km), while AMISR-14 detects small-scale irregularities (0.34 m). This study presents simultaneous observations of equatorial spread F (ESF) irregularities at 50–200 km scale sizes using the all-sky imager, at 3 m scale sizes using the JULIA (Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere) radar, and at 0.34 m scales using the AMISR-14 radar. We compare data from the three instruments on the night of 20–21 August 2014 by locating the radar scattering volume in the optical images. During this night no topside plumes were observed, and we only compare with bottomside ESF. AMISR-14 had five beams perpendicular to the magnetic field covering ~200 km in the east-west direction at 250 km altitude. Comparing the radar data with zenith ASI measurements, we found that most of the echoes occur on the western wall of the depletions with fewer echoes observed the eastern wall and center, contrary to previous comparisons of topside plumes that showed most of the echoes in the center of depleted regions. We attribute these differences to the occurrence of irregularities produced at submeter scales by the lower hybrid drift instability. Comparisons of the ASI observations with JULIA images show similar results to those found in the AMISR-14 and ASI comparison.

Comparative aeronomy: Molecular ionospheres at Earth and Mars

The ionospheres in our solar system vary not only in their electron densities, but also in the dominance of atomic versus molecular ions at their altitudes of peak plasma density. With the exception of Earths F-layer composed of atomic oxygen ions and electrons, all other planets have their peak ionospheric layers composed of molecular ions and electrons embedded in a dense neutral atmosphere. At Mars, both of its ionospheric layers have molecular ions, with the M1-layer at a lower altitude than the more robust M2-layer above it. The terrestrial ionosphere has a prominent region of molecular ions (the E-layer) below the dominant F-layer. In this paper, we explore the production and loss of molecular ion layers observed under the same solar irradiance conditions at Mars and Earth. We compare observations of M1 and M2 electron densities with terrestrial ionosonde data for the peak densities of the E- and F-layers during low, moderate and high solar flux conditions. The sub-solar peak densities of molecular ion layers have high correlations at each planet, as well as between planets, even though they are produced by separate portions of the solar spectrum. We use photo-chemical-equilibrium theory for layers produced by soft X-rays (M1 and E) versus the M2-layer produced by extreme ultraviolet (EUV) to identify the key parameters that cause similarities and differences. The yield of our comparative study points to the roles of secondary ionization and temperature dependent plasma recombination rates as areas most in need of further study at each planet.