Identification and Characterization of the Topside Bulge of the Venusian Ionosphere

TL;DR

This study analyzes Venus Express VeRa daytime electron-density profiles to characterize a persistent topside bulge in the Venusian ionosphere. An automated, gradient-based framework identifies and classifies bulge morphologies into Type 1 (distinct secondary peak), Type 2 (shoulder), and Type 3 (visible via residuals after Chapman fitting to the V2 peak), using Chapman fits to remove the V2 layer and residual analysis to quantify $h_{m,BL}$ and $N_{m,BL}$. The bulge appears in over $80\%$ of the 234 profiles and shows a strong dependence on solar zenith angle and solar activity, with Type 1 restricted to low latitudes and low $SZA$, and with bulge altitude $h_{m,BL}$ decreasing as $SZA$ increases. Solar wind dynamic pressure, adjusted for flow geometry, correlates with bulge occurrences, supporting a solar-wind–driven origin rather than purely photochemical processes, and suggesting that electron density enhancements may arise from solar wind interaction and thermospheric dynamics. These findings advance the understanding of Venus–solar wind coupling and have implications for future missions aiming to probe the topside ionosphere and its interaction with the solar wind.

Abstract

Venus, in the absence of an intrinsic magnetic field, undergoes a direct interaction between its ionosphere and the solar wind. Previous missions, including Mariner, Venera, and the Pioneer Venus Orbiter (PVO), reported a recurring localized increase in electron density, often termed a "bulge," at altitudes between 160 and 200 km. This study investigates this topside bulge using over 200 dayside electron density profiles derived from the Venus Radio Science experiment (VeRa) onboard the Venus Express (VEX). We employ an automated, gradient-based classification algorithm to provide a quantitative and reproducible method for identifying and categorizing the bulge morphology into three types. Type 1 profiles exhibit a distinct secondary peak above the main V2 layer. Type 2 profiles display a shoulder-like feature near the bulge altitude. Type 3 bulges are not visually apparent but can be identified through residuals obtained after subtracting a Chapman layer fit to the V2 peak. The bulge is detected in over 80\% of the analyzed profiles, with a higher occurrence during periods of low solar activity and at lower solar zenith angles (SZA). Type 1 morphologies are only observed at low latitudes (within $\pm 40^\circ$). The peak altitude of the bulge negatively correlates with SZA, suggesting that thermospheric cooling toward the terminator significantly influences the bulge altitude. The occurrence patterns and morphological characteristics indicate that the bulge is likely influenced by external drivers, such as solar wind interaction, rather than being solely a result of local photochemical processes.

Figures (14)

• Figure 1: A Venus Express VeRa radio occultation daytime electron density profile observed on 16 July 2009. The gray spheres represent the retrieved electron density, $N_{e}$. The black line shows the smoothed profile, $N_{s}$, obtained after 10 successive iterations using Equation \ref{['eq: smoothing']}. The vertical light gray line represents the $3\sigma$ uncertainty in the retrieval. Two ionospheric layers are marked: V2, the main peak formed by solar EUV ionization, and V1, formed by soft X-rays and secondary photoelectron impact ionization. The topside bulge located above the V2 layer is investigated in this work. The solar zenith angle during this observation was $22^\circ$.
• Figure 2: Spatial distribution of all VeRa profiles on a Venus latitude--longitude map. The gap between $40^\circ$ and $60^\circ$ latitude arises from the orbital geometry of Venus Express (VEX), which follows a highly elliptical orbit. Because the pericenter of VEX is close to the north pole, most profiles at high northern latitudes do not extend above $350~km$ and are therefore excluded from our analysis.
• Figure 3: Spatial distribution of selected 234 electron density profiles in the Venusian ionosphere used in this study. Observations are primarily concentrated in the equatorial and southern hemisphere regions. High, moderate, and low solar activity levels are indicated by red, green, and blue, respectively. Symbols represent morphologies: star for Type 1, cross for Type 2, open circle for Type 3, and closed circle for Type 4.
• Figure 4: Examples of the three bulge morphologies identified in VeRa electron density profiles using the Chapman fit method (Section \ref{['Electron density']}: (a) Illustration of different boundary selections for Chapman fitting; (b) Type 1 — The bulge appears as a distinct secondary peak located above the main V2 layer; (c) Type 2 — The bulge shows as a shoulder-like enhancement near the V2 peak; and (d) Type 3 — No visually distinct bulge is present, but the residual electron density ($N_{res}$) obtained after subtracting a Chapman fit ($N_{chap, V2}$) from the smoothed profile ($N_{s}$) indicates a layer above the V2 peak. Profiles that do not contain a valid bulge based on the defined criteria are not categorized. Here, $W_{\mathrm{bulge}}$ represents the width of the bulge, and $m_{\mathrm{BL,top}}$ and $m_{\mathrm{BL,bottom}}$ denote the average gradients of the $N_{S}$ above and below $N_{m,\mathrm{BL}}$, respectively, within the width of the bulge (see Section \ref{['Electron density']}).
• Figure 5: Panels (a) and (b) show the solar wind number density ($n_{SW}$) and velocity ($v_{SW}$) at Venus derived from VEX ASPERA-4. Panel (c) shows the solar wind dynamic pressure (eq. \ref{['eq: pristin_SW']}). Panel (d) shows the solar activity index $F_{10.7P}$ scaled to Venus. The gray bands mark the occultation seasons (occ 8 was not carried out due to operational limits).