OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Space Technology
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OBJECTIVE: The objective of this project is to deliver a predictive model of sporadic E occurrence. The model would allow the user to see a "weather" forecast for sporadic E statistics for a given time and place. The forecast would run out hours, days, weeks, or even months into the future with uncertainty increasing with forecast length. Sporadic E statistics might include percent of day (or other time unit) with blanketing or sporadic E as a function of frequency. The model could also provide information on the expected peak frequency of Sporadic E. Other parameters, such as a yet to be defined "severity" metric could also be provided. The model should be data-assimilative and global. Possible data sources included ionograms, radio occultation measurements, high altitude wind measurements, and sporadic E seed population data. Other data sources are also possible.
DESCRIPTION: Historically, Earth’s ionosphere is separated into 3 distinct layers, namely the D (50-90 km), E (90 – 150 km), and F (150 – 500 km) layers. It is well known that the plasma in these layers impacts propagation at radio frequencies (RF). When the ionosphere is smoothly and slowly varying, it is generally straight forward to model these impacts. The ionosphere, however, is not always in a behaved and well-defined state. Irregular ionospheric structures severely alter RF propagation or produce scintillation. Numerous mechanisms exist that cause irregular structure in the ionosphere. One such mechanism is Sporadic E, which can be described as an unusually dense E layer that sporadically appears for relatively short periods of time. Sporadic E can sometime be so dense that it completely blocks skywave propagation to and from the F layer. When this blocking occurs, the phenomenon is often referred to as blanketing E. It is generally accepted that Sporadic E forms when long lasting meteoritic ions are pushed into a thin, dense layer due to wind shear interactions and the geomagnetic field. Sporadic E occurrence is modulated on timescales from years to hours. Annually, sporadic E occurs much more frequently in the summer. Daily, it is modulated by the semi-diurnal tides, which trigger a descending horizontalwind shear in the thermosphere twice a day. These descending wind shears have been observed in wind measurements as well in measurements of descending sporadic E layers. Additionally, it has been shown that Sporadic E is modulated by quasi-periodic planetary waves with periods ranging from a few days to a few weeks (Haldoupis and Pancheva 2002). It is not exactly known how the planetary waves produce the sporadic E modulation, but the effects are easy to measure. Ionosondes, which measure the electron density as a function of altitude, are the primary tool used for detecting sporadic E. In fact, the term sporadic E is a descriptive term used to describe ionograms (ionosonde measurements) that are affected by the overly dense E layer phenomenon. Since sporadic E is such a thin layer, the only two measured characteristics are the height and peak plasma frequency. These two parameters could then be input into a RF propagation model to determine the geometric impacts. Based on recent observations, the length scales of dense sporadic E structures are on the order of 10s to 100s of km, depending on the orientation. The goal of this topic, however, is not to characterize the exact morphology of sporadic E, rather we intend to predict the occurrence rate and characteristics of sporadic E at any point on Earth. Since it is difficult to globally monitor the drivers of sporadic E (thermospheric winds and meteoritic seed populations), a physics-based forecast model may be impractical. The impacts of sporadic E, however, are regularly observed by numerous instruments, including ionosonde networks and GNSS radio occultation (RO) capable satellite constellations. Since Sporadic E is subject to global weather patterns, spatial and temporal correlation functions could adequately describe future occurrence. For instance, observations in western US today could predict observations in eastern US tomorrow, and Europe a week later. Existing global sensor networks likely provide the necessary tools to build an empirical, predictive model. Such a model currently does not exist. We seek a global forecast model that leverages existing data sources. Preference is given to publicly available data but exceptions can be made. The model should act like a weather forecast for scattered thunderstorms, where the product is a percent probability of occurrence and overall severity. The model should focus on the characteristics of Sporadic E that impact high frequency (HF; 3-30 MHz) propagation but other aspects (e.g. scintillation) can be considered as well. The model will be validated by the performer and customer by comparing predicted Sporadic E levels at Ionosonde sites or other instrument sites with actual measurements.
PHASE I: Phase I will demonstrate the feasibility of using existing data sources for use in a Sporadic E forecast model. Specifically the demonstration would include a global Sporadic E occurrence/severity study using available Ionosonde data and possibly other data sources. The feasibility of a potential model will be determined by the measured temporal and spatial correlations between stations. If a strong enough correlation exists on planetary wave size scales then the model will likely be deemed feasible by the TPOC.
PHASE II: Develop a data-assimilative forecast model based on the results of phase I. The model should be similar to a weather forecast model where the user can get the percent chance for Sporadic E for a given place. The forecast would stretch out days to weeks depending on how far in advance the information is statistically meaningful. The forecast model should be validated using existing data sources. New data sources through AFRL may also become available for validation. The model will be compared to median climatology estimates to determine its usefulness as a forecast tool.
PHASE III DUAL USE APPLICATIONS: There are a number of government and private organizations that rely on accurate predictions of RF interactions with the space environment. Furthermore, the proposed model has potential use within the US Space Force Space Systems Command as well as Air Combat Command, the US Navy, and other DoD and Title 50 organizations.
- Haldoupis, C. and Pancheva, D. (2002) Planetary waves and midlatitude sporadic E layers: Strong experimental evidence for a close relationship, JGR Space Physics, 107, A6;
- Tang, Q., Zhao, J., Yu, Z., et al. (2021) Occurrence and Variations of Middle and Low Latitude Sporadic E Layer Investigated With Longitudinal and Latitudinal Chains of Ionosondes, Space Weather, 19, 2;
- Shinagawa, H., Tao, C., Jin, H. et al. (2021) Numerical prediction of sporadic E layer occurrence using GAIA, Earth Planets and Space, 73, 28
KEYWORDS: Sporadic E; ionospheric model; ionosonde