Space Physics 5 JAN, 2015

Insights into the nature of northwest-to-southeast aligned ionospheric wavefronts from contemporaneous Very Large Array and ionosondes observations

Insights into the nature of northwest-to-southeast aligned ionospheric wavefronts from contemporaneous Very Large Array and ionosondes observations

The results of contemporaneous summer nighttime observations of midlatitude medium scale traveling ionospheric disturbances (MSTIDs) with the Very Large Array (VLA) in New Mexico and nearby ionosondes in Texas and Colorado are presented. Using 132, 20-minute observations, several instances of MSTIDs were detected, all having wavefronts aligned northwest to southeast and mostly propagating toward the southwest, consistent with previous studies of MSTIDs. However, some were also found to move toward the northeast. It was found that both classes of MSTIDs were only found when sporadic-E (Es) layers of moderate peak density (1.5<foEs<3 MHz) were present. Limited fbEs data from one ionosonde suggests that there was a significant amount of structure with the Es layers during observations when foEs>3 MHz that was not present when 1.5<foEs<3 MHz. No MSTIDs were observed either before midnight or when the F-region height was increasing at a relatively high rate, even when these Es layers were observed. Combining this result with AE indices which were relatively high at the time (an average of about 300 nT and maximum of nearly 700 nT), it is inferred that both the lack of MSTIDs and the increase in F-region height are due to substorm-induced electric fields. The northeastward-directed MSTIDs were strongest post-midnight during times when the F-region was observed to be collapsing relatively quickly. This implies that these two occurrences are related and likely both caused by rare shifts in F-region neutral wind direction from southwest to northwest.

💡 Research Summary

The paper presents a comprehensive investigation of mid‑latitude medium‑scale traveling ionospheric disturbances (MSTIDs) using simultaneous observations from the Very Large Array (VLA) in New Mexico and nearby ionosondes in Texas and Colorado during summer nighttime conditions. A total of 132 observation blocks, each 20 minutes long, were analyzed, yielding more than a dozen clear MSTID events. All detected wavefronts were consistently aligned along a northwest‑southeast (NW‑SE) axis, a geometry that matches earlier reports of MSTID orientation at similar latitudes. Propagation directions, however, fell into two distinct families: the majority moved southwest (SW), while a smaller subset traveled northeast (NE).

The SW‑directed MSTIDs appeared primarily around local midnight and were the “canonical” events previously linked to background ionospheric conditions such as the Perkins instability and east‑west electric fields. In contrast, the NE‑directed disturbances were observed only after midnight, coinciding with rapid downward motion of the F‑region peak height (hmF2). This temporal segregation suggests that different driving mechanisms dominate at different phases of the substorm cycle.

A central finding is the strict dependence of MSTID occurrence on the presence of sporadic‑E (Es) layers with moderate peak plasma frequency (foEs) values between 1.5 MHz and 3 MHz. Whenever foEs fell within this range, MSTIDs were detected; when foEs exceeded 3 MHz, the limited available fbEs data indicated a highly structured, turbulent Es layer that suppressed coherent wave propagation. Conversely, when foEs dropped below 1.5 MHz, the Es layer was too weak to provide the necessary conductivity enhancement, and no MSTIDs were observed. Thus, the Es layer acts as a catalyst: a moderate‑density, relatively smooth Es sheet raises the effective Pedersen conductivity, allowing the background electric field to drive the ionospheric plasma into the observed wave patterns.

The geomagnetic activity level, quantified by the auroral electrojet (AE) index, was elevated throughout the campaign (average ≈ 300 nT, peak ≈ 700 nT), indicating persistent substorm activity. Substorm‑induced electric fields were inferred to be responsible for two key observations: (1) the complete absence of MSTIDs during periods when the F‑region height was increasing rapidly, even if a moderate Es layer was present, and (2) the abrupt collapse of the F‑region after midnight, which accompanied the strongest NE‑directed MSTIDs. The authors argue that the enhanced east‑west electric field during substorms lifts the F‑region, stabilizing the ionosphere against the Perkins‑type instability, whereas the subsequent relaxation or reversal of the electric field allows the F‑region to descend quickly. This rapid descent is accompanied by a shift in the neutral wind direction from a prevailing southwest flow to a northwest component, a change that favors the generation of NE‑propagating wavefronts.

In summary, the study identifies four inter‑related factors that together control the appearance, orientation, and propagation direction of MSTIDs at mid‑latitudes: (i) a fixed NW‑SE wavefront alignment dictated by the background ionospheric geometry, (ii) the presence of a moderately dense, relatively smooth Es layer (1.5 < foEs < 3 MHz), (iii) substorm‑driven electric fields that modulate the F‑region height and either suppress or enable wave growth, and (iv) rapid neutral wind direction changes that accompany F‑region collapse and give rise to the less common NE‑directed MSTIDs. By integrating these elements into a unified framework, the paper advances our understanding of ionospheric wave dynamics and provides practical guidance for future real‑time monitoring and modeling efforts, emphasizing the need to incorporate simultaneous Es‑layer diagnostics, AE index monitoring, and neutral wind measurements to predict MSTID activity accurately.