Relationship Between Presunset Wave Structures and Interbubble Spacing: The Seeding Perspective of Equatorial Plasma Bubble

Abstract

In this paper, we study on the precursor seed structures well before the prospective occurrence of equatorial plasma bubble (EPB). We use the scanning mode observations made by the 30‐MHz Gadanki Ionospheric Radar Interferometer for estimating interbubble spacing and high time resolution observations made by a collocated digisonde for inferring horizontal wavelength of wave‐like variations prior to the onset of EPB. Horizontal wavelength of wave‐like variations has been inferred from plasma density variations and height variations of isodensity contours; both show consistent results. Remarkable agreement has been found between the estimated horizontal wavelength and interbubble spacing, which demonstrates the potential of digisonde in providing relevant seed structure for a prospective EPB formation. Further analysis based on observational results and numerical simulation of collisional interchange instability shows that while the eventual formation of EPB has a strong bearing on the height of the F layer and density gradient scale length, the growth perspective of the EPB depends on the seed wavelength. These results are presented and discussed keeping the future prospective of EPB forecasting/nowcasting.

On the Assessment of Day‐To‐Day Occurrence of Equatorial Plasma Bubble

“…It may be mentioned that due to the lack of concurrent wind measurements, we have assumed the background wind to be zero. Although this is not realistic, we have found in our earlier study (Das et al., 2020) that the inferred horizontal wavelengths can be used quite satisfactorily. For the present purpose, the horizontal wavelength is estimated as (Fritts & Alexander, 2003): where, is the horizontal wavelength of the gravity wave‐like variations; T b is the Brunt Väisälä period; and H is the scale height of the neutral gas at 300 km.…”

“…It may be mentioned that due to the lack of concurrent wind measurements, we have assumed the background wind to be zero. Although this is not realistic, we have found in our earlier study (Das et al, 2020) that the inferred horizontal wavelengths can be used quite satisfactorily. For the present purpose, the horizontal wavelength is estimated as (Fritts & Alexander, 2003):…”

“…For characterizing gravity wave-like variations, we have used both height and electron density variations. More details on similar analysis using the GIRI and DPS-4D observations can be found in Das et al (2020).…”

“…More details on similar analysis using the GIRI and DPS‐4D observations can be found in Das et al. (2020).…”

“…In a recent paper (Das et al., 2020), we analyzed background ionospheric variations prior to the occurrence of EPB in terms of wave structures and their linkage to EPB. More specifically, we found that the wavelength of background wave structures inferred from ionosonde observations are well correlated with the EPB spacing.…”