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Abstract The D‐region ionosphere (6090 km) plays an important role in long‐range communication and response to solar and space weather; however, it is difficult to directly measure with currently available technology. Very low frequency (VLF) radio remote sensing is one of the more promising approaches, using the efficient reflection of VLF waves from the D‐region. A number of VLF beacons can therefore be turned into diagnostic tools. VLF remote sensing techniques are useful and can provide global coverage, but in practice have been applied to a limited area and often on only a small number of days. In this work, we expand the use of a recently introduced machine learning based approach (Gross & Cohen, 2020,https://doi.org/10.1029/2019JA027135) to observe and model the D‐region electron density using VLF transmitting beacons and receivers. We have extended the model to cover nighttime in addition to daytime, and have applied it to track D‐region waveguide parameters, h’ and, over 400 daytimes and 150 nighttimes on up to 21 transmitter‐receiver paths across the continental US. Using an exponential fit, h’ represents the height of the ionosphere andrepresents the slope of the electron density. Using this data set, we quantify diurnal, daily and seasonal variations of the D‐region ionosphere for both daytime and nighttime D‐region ionosphere. We show that our model identifies expected variations, as well as producing results in line with other previous studies. Additionally, we show that our daytime predictions exhibit a larger autocorrelation at higher time lags than our nighttime predictions, indicating a model with persistence may perform better.
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D region Ionospheric Imaging Using VLF/LF Broadband Sferics, Forward Modeling, and Tomography
The D region of the ionosphere (60-90 km) is very important for a variety of radio science applications. It is utilized in important applications as it reflects Very Low Frequency (VLF, 3-30 kHz) waves, ranging from navigation to communication to lighting geolocation. However, despite the large body of D region applications and research, we still know relatively little about the underlying structure. This is in large part due to the difficulty of making direct measurements, which is why many workers have utilized the same VLF waves to perform remote sensing studies. However, noticeably absent in the field, is a thorough study which attempts to recover a map of the D region. We describe possible methods to utilize the broadband VLF emissions from lightning and a receiver network to obtain a 2D ionospheric map using methods from Compressed Sensing (CS) and Computed Tomography (CT).
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- PAR ID:
- 10341631
- Date Published:
- Journal Name:
- 25th International Lightning Detection Conference
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We present a new four‐parameter model of theD‐region (60–90 km) ionospheric electron density, useful in very low frequency (VLF, 3–30 kHz) remote sensing. VLF waves have a long history of use to indirectly inferD‐region conditions, as they reflect efficiently and thus are sensitive to small changes in the electron density. Most historical efforts use VLF observations along with a forward model of theD‐region and VLF propagation. The ionospheric assumptions in the forward model are altered until the output matches the observation. The most commonD‐region model, known as the Wait‐Spies ionosphere, takes the electron density as exponentially increasing with altitude and specifies a height and steepness. This model was designed to capture the VLF propagation variations evident at a single frequency. The realD‐region is likely more complex. The limited number ofD‐region rocket passes that have previously been compiled tend to show the existence of a “ledge” somewhere between 70 and 90 km. Broadband VLF signals emitted from lightning allows a more sophisticated parametrization. Using carefully averaged amplitudes and phases of VLF sferics, we formulate a more general four‐parameterD‐region model that includes a ledge discontinuity. Using lightning‐emitted VLF observations along with a theoretical model, we find that this model better describes the ionosphere during the daytime. During the ambient nighttime and during a solar flare the two‐parameter ionosphere may be sufficient, at least for the purposes of calculating broadband VLF propagation, since the ledge either weakens or moves outside the altitude range of VLF sensitivity.more » « less
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