CD1 – Solar and lightning forcing of the ionospheric D-layer: recent developments & results
Talks
CD1 Thu 7/11 14:15-15:15, room C2D – Almedina
Author(s): Steven Cummer, Yunjiao Pu, Yongze Jia, Zilong Qin, Fanchao Lyu, Mingli Chen
Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA; Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA; Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA; Guangdong-Hong Kong-Macao Greater Bay Area Weather Research Center for Monitoring Warning and Forecasting (Shenzhen Institute of Meteorological Innovation), Shenzhen, China; State Key Laboratory of Severe Weather & CMA Key Laboratory of Lightning, Chinese Academy of Meteorological Sciences, Beijing, China; Department of Building Environment and Energy Engineering, The Hong Kong Polytechnic University, Hong Kong, China
Abstract: Understanding the state and variability of the D region of the ionosphere is increasing in relevance due to expanding applications in long-distance radio positioning and timing, over-the-horizon sensing, and detection of upper atmospheric perturbations produced by a wide range of natural and man-made sources. Yet it remains difficult to measure thanks to its physical inaccessibility and the relatively low radio frequencies required to obtain reflections from the D region. Broadband and narrowband VLF and LF signals from lightning and man-made transmitters, respectively, have been used with much recent success to probe D region variability. But in many ways the ideal source signal is a controlled LF pulse. Such transmitters still exist in the form of pulsed 100 kHz Loran C transmitters that provide positioning and timing capabilities and are operating with increasing frequency in several locations across the world including the US. We have recorded multiple years of nearly continuous Loran C data from multiple midlatitude transmitters on several continents. With judicious signal averaging the amplitude and phase of the one-hop and two-hop skywaves can measured continuously every 5-10 seconds. The measurements show local D region variability on time scales of a few minutes to many days. In some cases these variations can be attributed to specific sources including thunderstorms, lightning, and even atmospheric gravity waves produced by the Hunga Tonga volcanic eruption. We will present a summary of the capabilities of this measurement and examples of the perturbations produced by lightning, space weather, and other sources.
Author(s): Pauline Teysseyre, Carine Briand, Morris Cohen
LESIA – Observatoire de Paris, PSL; LESIA – Observatoire de Paris, PSL; Georgia Institute of Technology
Abstract: The ionospheric D-layer is responsible for a large part of the HF absorption; thus, it is crucial to monitor it. Due to its altitude range and weak electron density, the most common monitoring method relies on the propagation of Very Low Frequency (VLF) waves.
We aim to understand the time evolution of the perturbations in the electron density in the D-layer for different solar flares (> M1). Particular emphasis will be given to the difference between very short forcing (flares of a few tens of minutes), longer ones (half an hour or more) and multiple forcing.
We present here the analysis from data from two AWESOME instruments (Cohen et al., 2018), situated in Nançay (Sologne, France) and La Réunion (France, near Madagascar). A variety of propagation paths are used, with lengths varying between 900 and 7000 km. Some of those propagation paths are oriented in the North-South direction, while others lie in the East-West direction.
The amplitude and phase at the receiver are modelled, using the Longwave Mode Propagator (LMP, Gasdia & Marshall, 2021). The electron density in the D-layer is inferred by comparing modelled and measured amplitude and phase.
Author(s): Tamal Basak, Sayak Chakraborty, Sourav Palit
Indian Centre for Space Physics Kolkata; Indian Centre for Space Physics Kolkata; Indian Centre for Space Physics Kolkata
Abstract: We take a straightforward approach to study the D-region ionosphere by solving the ‘electron continuity equation’ during the occurrence of solar flares. In our study, we analyze the ionization rate of D-region by X-ray. We study the D-region effective response time delay (Δt) for different classes of solar flares and report that Δt has significant seasonal dependency and dependency on geographic location where it is measured from. We choose a few hundreds of solar flares from different classes occurred on similar season of different years and computed their respective Δt’s and do a statistical analysis. We report that Δt has a tendency of decaying with increasing strength of solar flare. We checked a number of statistical parameters to study the overall behavior of Δt. We provide necessary explanations.
Author(s): Tobias Verhulst, Veronika Barta, Attila Buzás
STCE – Royal Meteorological Institute, Belgium; HUN-REN Institute of Earth Physics and Space Science, Sopron, Hungary; HUN-REN-ELTE Space Research Group, Budapest, Hungary
Abstract: Continuous, near real-time observations of HF radio absorption in the D layer are becoming important for various application, for example with regards to HF radio communication with air-planes. Various methods exist for measuring ionospheric absorption, often being employed for different frequency ranges. One technique—traditionally referred to as “A2”—involves measuring the level of extraterrestrial radio noise. The major source of such noise in the HF and VHF bands is the Milky Way galaxy and varies smoothly throughout the day.
Extraterrestrial radio noise at frequencies below F2-layer critical frequency will of course not reach the Earth’s surface. Noise at higher frequencies can reach the surface, but will be absorbed to a greater or lesser extend in the D region. Once the normal, quiet time diurnal variation of the noise at a given frequency has been established, the deviations from this curve can be used to monitor enhancements of ionisation in this region.
Usually, such measurements are performed by dedicated riometer instruments. Riometers are mostly installed in high-latitude regions, and data from other areas tends to be sparse. Here, we will present an attempt to use existing ionosondes in a “riometer-like” operating mode to acquire A2-type measurements of ionospheric absorption. We use data from Digisonde DPS4D instruments, which have low internal noise levels and stable receiver gains and can make observations with a narrow bandwidth. We concentrate on the European mid-latitude region, where data from several observatories is available.
We use passive measurements, without ionosonde transmissions, at various frequencies above foF2 and up to 30 MHz. For each frequency range, the quiet-time diurnal variation of the radio noise is established. We then look at the deviations from this diurnal pattern induced by various solar flares that occurred in the spring of 2024.
Posters
Posters II Display Thu 7/11 – Fri 8/11, room C1A – Aeminium
Authors in attendance: Thu 7/11 10:15–11:30, 15:15-16:15; Fri 8/11 10:15–11:30
Author(s): Oswald Didier Franck GRODJI
Université Félix Houphouët Boigny
Abstract: We investigated the impact of solar flares on the horizontal (H), eastward (Y) and vertical (Z) components of the geomagnetic field during solar cycles 23 and 24 using a chain of magnetometers measurements on the sunlit side of the Earth. We examined the relation between sunspot number and solar flare occurrence of various classes during both cycles. We obtained a positive linear relation with correlation coefficient of 0.93/0.97, 0.96/0.96 and 0.60/0.56 for C-class, M-class and X-class flare, respectively during SC23/24. The three geomagnetic components of the geomagnetic field exhibited a peak few minutes after the solar flare occurrence. Generally, the magnetic crochet of the H component was negative at mid-latitudes in both hemispheres and positive at low-latitudes. The peak amplitude of solar flare effect (sfe) on the various geomagnetic components depended of many factors including the local time at the observing station, the solar zenith angle, the position of the station with respect to the magnetic equator and the intensity of the flare. Thus, these peaks were stronger for the stations around the magnetic equator and very low when the geomagnetic field components were close to their night values. Both cycles presented similar monthly variations with the highest sfe value (ΔHsfe = 48.82 nT for cycle 23 and ΔHsfe = 24.68 nT for cycle 24) registered in September and lowest in June for cycle 23 (ΔHsfe =8.69 nT) and July for cycle 24 (ΔHsfe =10.69 nT). Also the sfe was generally higher in cycle 23 than in cycle 24.
Author(s): Tamal Basak, Rupak Mukherjee
Indian Centre for Space Physics, Kolkata, India; Department of Physics, Sikkim University, Sikkim, India
Abstract: Study of the propagation effects of sub-ionospheric very low frequency (VLF) signal is one of the established techniques to monitor the physical properties of the lower ionosphere. In this work, we present the VLF signal characteristics as obtained from numeical techniques using standard radio propagation codes for a possible VLF signal receiving station at Sikkim University, Gangtok, India (27.3N, 88.6E) (SUG-VLF). We obtained notable diurnal profiles each for VLF signal coming from JJI, VTX, NWC and JJY transmitters. Furthermore, we computed a finite VLF signal perturbation during solar flare effects for all the mentioned transmitter-receiver propagation paths. We report some initial results on the D-region electron density profiles (Ne) across the propagation paths. We provide some comparisons among numerical results obtained for the possible SUG-VLF site and other already established VLF sites in India. We provide the possible explanation about the added importance VLF-ionosphere interaction study at high altitude Himalayan region.
Author(s): Sayak Chakraborty, Tamal Basak
Indian Centre for Space Physics, India; Indian Centre for Space Physics, India
Abstract: The primary source of energy governing the D-region ionospheric dynamics and evolution is mostly the solar Lyman-α radiation. The penetration and hence, the ionization by solar radiation at lower altitudes (~75 km) is only possible for solar radiation of wavelengths between 110-121.6 nm. During low solar activity period the X-ray emission level can hardly penetrate to the D-region, resulting Lyman-α radiation remaining the primary source of ionization for D-region ionosphere. The scenario becomes different during high solar activity time when X-ray emission contributes equally or even more in some cases in this ionization process. We seek to model the long-term effect on the D-region ionosphere for the entire Solar Cycle-24 (C24). As the solar cycle passes through both low and high solar activity phases, we consider the effects of both (i) solar Extreme Ultraviolet (EUV) and (ii) soft X-ray spectra in our model. We compute and compare the collective ionization rates (q’s) for these two emission and compute electron density (Ne,model) by solving the ‘D-region electron continuity equation’ using numerical techniques. We compare Ne,model with its IRI-2020 counterpart (Ne,iri). For better understanding of the solar cycle dependency, we correlate Ne,model with monthly average sunspot number (SPN). We report a significant solar cycle variation in Ne along with Gnevyshev’s dual peak in Ne,model curve during solar maxima period as observed in SPN and solar EUV and X-ray flux as well.
Author(s): Neil R. Thomson, Craig J. Rodger, Mark A. Clilverd
Department of Physics, University of Otago, Dunedin, New Zealand; Department of Physics, University of Otago, Dunedin, New Zealand; British Antarctic Survey (UKRI-NERC), Cambridge, UK
Abstract: Electron density characteristics of the Earth’s ionospheric D-region, in the height range ~55-90 km, have been determined using phase and amplitude measurements of man-made, very low frequency (VLF) radio propagation in the Earth-ionosphere waveguide. Time variations of the ‘Wait’ height and sharpness parameters, H’ and Beta, were identified through long-wave propagation modelling of VLF observations recorded over many years, and on many transmitter-receiver paths. This work was primarily undertaken by Neil Thomson, aided and abetted over the years by Craig Rodger and Mark Clilverd. Since 2010 Neil Thomson has led ten key papers describing the variation in H’ and Beta over a wide range of relevant propagation situations, i.e., during the day, during the night, and over low-mid-high latitudes. Interpretation of the results illuminates the geophysical effects controlling D-region electron concentration profiles, including the dominant role of solar Lyman alpha at low and mid-latitudes, and the greater role of galactic cosmic rays at increasingly higher mid-latitudes.
In the most recent of the Thomson papers (in review), Wait parameters have been determined through dawn and dusk for the first time. The analysis provides H’ and Beta values to constrain D-region modeling efforts, thereby extending the capabilities of VLF propagation monitoring for geophysical phenomena such as lightning, solar flares, and energetic particle precipitation. This talk summarises the recent work, focusing on dawn and dusk observations, and putting the resultant H’ and Beta variations into context with day and night values, over a range of latitudes.
Author(s): Vida Žigman, János Lichtenberger
University of Nova Gorica, Nova Gorica, Slovenia; Space Research Group, Dpt. of Geophysics and Space Sciences, Eötvös University, Budapest, Hungary
Abstract: Solar flares are regularly detected in different portions of the X-ray and EUV spectra by a myriad of sensors aboard space missions, to mention just few: the historic GOES, SDO, LYRA… Though the effect of the flare on the D-region is most effective in the soft X-ray range, flares have also a hard X-ray component and can even have a gamma part of the spectrum, as was the case of the X7.7 flare of 2012_03_07 detected by the Fermi Large Area Telescope in the gamma range with energies > 100MeV.
The terrestrial VLF technique of detecting solar impact on the Earth closest ionosphere rests on VLF (<30 kHz) transmitters (Tx), which are already there for military reasons, but are as well a fortuitous tool for the exploration of the D-region. This is achieved by vast networks of VLF receivers (Rx), constantly consolidated, practically embedding the Earth, e.g. AARDDVARK, GIFDS, SAVNET, WWLLN, etc… By way of VLF propagation along the Earth-ionosphere waveguide ionization changes in the D-region can be detected, regardless of their particular origin, apart from flares, also e.g. SEPs, lightning and LEPs and even GLE events.
Here we address the VLF study of solar flares, concentrating on the flare induced disturbance of the amplitude and phase time profiles, as these are detected by different Rx on different Tx-Rx sunlit paths. There is a regular feature in the recordings: the amplitude and phase extreme disturbance is delayed with respect to the maximum of the flare as detected by GOES satellites in the soft X-ray range (0.1, 0.8) nm. This is a manifestation of the sluggishness of the Ionosphere in accommodating to new flare induced conditions. Recently, the time delay in amplitude, in particular, has been recognized as an important parameter in diagnostic the flare impact.
In the current study we widen the research so as to analyse both the amplitude and phase time delays on equal terms, to accomplish accurate ionospheric modelling. We also consider the appearance of negative time delay found with respect to the soft X-ray emissions for major X-class flares: the extreme VLF amplitude and/or phase precede the irradiance maximum. This is taken as an indication that some radiation other than the soft X-rays is the source of ionization in the initial stage of the flare, since harder X-rays reach the maximum before the soft X-rays. We aim to confirm this assumption by relating the ground-based measurements of VLF amplitude and phase perturbations to the space recordings of both solar soft and hard X-ray irradiance as given by GOES satellites. On the basis of continuum theory and aeronomy a physics-based model for the reconstruction of the D-region electron density enhancements is used. We model the effective ionization and recombination coefficients for each flare in particular by using the FISM2 solar spectral irradiance model. We illustrate the outcome of the analysis, by addressing historic flare data, from a series of strong eruptive events in January 2005 and the recent most intense flares of May 2024.