ORigin determination and improved detectAbility of Celestial-to-Local phEnomena by the VLF technique

Project Information

ORACLE

Grant agreement ID: 101019319

DOI

10.3030/101019319

Project terminated on 31 December 2023

EC signature date 9 March 2021

Start date 5 August 2022

End date 4 August 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

UNIVERSITY OF BATH
United Kingdom

Periodic Reporting for period 1 - ORACLE (ORigin determination and improved detectAbility of Celestial-to-Local phEnomena by the VLF technique)

Reporting period: 2022-08-05 to 2024-08-04

Summary of the context and overall objectives of the project

The propagation of Very Low Frequency (VLF: 3–30 kHz) radio waves can be used to remotely monitor two different regions of the Earth systems. One of them is the lower ionosphere, an ionized region of the Earth’s upper atmosphere located between 60 and 90 km in altitude. The other is the magnetosphere. ORACLE uses this VLF remote sensing technique to investigate the short- and long-term variation of the lower boundary of the ionospheree which acts as a monitoring screen for phenomena originating in the Earth’s atmosphere (e.g. ozone shadowing) and in space (e.g. gamma-ray bursts from celestial objects).

The lower boundary of the ionosphere is also known as the ignorosphere because it is the least studied region of the atmosphere. At the same time, this region is where space and space weather processes couple into the Earth’s atmosphere. Thus, ORACLE’s aim is to provide new knowledge on the physical process and conditions in the lower boundary of the ionosphere.

Humankind owes its origin, evolution and present existence to the Earth and its surrounding space, from the atmosphere to the limits of the universe. Yet, this very system poses several threats to society, from galactic gamma ray bursts to solar storms, from ozone layer depletion to extreme weather changes. In fact, all these hazards are able to affect the ionosphere and the magnetosphere.

ORACLE's scientific objectives are:
1) determine whether the day-to-day variability of the VLF signal during sunrise can be explained by the stratospheric ozone variability at its upper boundary,
2) improve the detection of celestial gamma-ray bursts, known as the most energetic phenomena in the universe.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

ORACLE uses VLF data sets from two different networks that record radio signals continuously since 2006. One of the networks has VLF receivers located in Latin America and the other has VLF receivers around the globe, particularly at middle- to high-latitude regions. This VLF data is interpreted to reflect the variability of the lower ionosphere.

The analysis was divided according to latitudinal regions (low-, middle-, and high-latitudes) using a set of VLF transmitters and receivers located in one of these regions. The VLF daily variability during sunrise was compared to the stratospheric ozone variability at its upper boundary. We found that the stratospheric ozone variability at low latitudes cannot solely produce the shadowing effect. This suggests that other physical processes are involved. Similarly, we found that the day-to-day variability of the VLF signal during sunrise cannot be explained exclusively by the day-to-day variability of the stratospheric ozone at its upper boundary.

One-to-one ionospheric responses to celestial gamma ray bursts events with durations longer than 1 min were selected. The analysis was conducted for both, daytime and nighttime conditions. We found that no significant ionospheric changes can be observed for gamma ray bursts of short duration (< 2 min) and mid-size events. This study concludes that only extreme events, which occurs rarely, can produce ionospheric disturbances that are remotely sensed by the propagation of VLF waves.

ORACLE presented these results of the action on 8 scientific conferences, plus 5 seminars as outreach activities to the general public and young students from Latin America and Europe.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

ORACLE findings constitute a progress beyond the state of the art. These are: i) improving the scientific understanding of the ionospheric response to celestial gamma ray bursts, and ii) providing new knowledge on the ozone shadowing effect on the lower ionosphere and the latitudinal dependence of this effect. The impact of ORACLE has two main different aspects. For the scientific community, ORACLE contributes to the understanding of both, the lower ionosphere dynamics, and the atmosphere-ionosphere coupling. For the general public, ORACLE reached out to the younger generations (school and university students) from Europe and Latin America to popularize its science. This younger generation has the potential to become the next scientific elite of Europe.

Field campaign in South Africa, measuring VLF radio waves