Multi-instrument constraints on a hemispherically asymmetric positive ionospheric storm in the 60-180 deg E sector during the 12-13 November 2025 geomagnetic storm

Abstract

Geomagnetic storms drive complex ionospheric responses through coupled electrodynamic and thermospheric processes, yet attributing storm-time TEC perturbations to specific mechanisms remains challenging. We investigate the ionospheric response to the 12-13 November 2025 intense geomagnetic storm (Dst minimum = -214 nT) in the 60-180 deg E sector using a coordinated multi-instrument dataset comprising JPL GIM TEC, dense regional GNSS networks, continuous BeiDou GEO links, COSMIC-2 radio occultation, ground ionosondes, Swarm in-situ electron density, HF Doppler soundings, and TIMED/GUVI thermospheric composition observations. The observations reveal a dayside-dominant positive TEC storm with pronounced hemispheric asymmetry, where Northern Hemisphere mid-to-low latitudes exhibit stronger and longer-lasting enhancement than the Southern Hemisphere. Joint analysis of radio occultation, ionosonde, and Swarm data indicates that the enhancement is density-dominated with NmF2 and foF2 increases but with no coherent, sector-scale peak-height uplift in hmF2 or h'F2, posing challenges for uplift-only electrodynamic interpretations. Coherent large-scale traveling ionospheric disturbances propagate across the equator during UT 1-6, while HF Doppler oscillations maximize later during UT 6-24, revealing a timing offset between integrated TEC responses and reflection-height dynamics. Southern Hemisphere O/N2 ratio depletion observed by TIMED/GUVI provides compositional context consistent with the faster positive-phase decay there, although concurrent Northern Hemisphere GUVI coverage is limited during this interval. These findings highlight the value of multi-observable diagnostics for developing testable constraints on storm-time mechanisms and improving sector-specific space weather nowcasting capabilities.

Figures (12)

• Figure 1: Interplanetary driving conditions and geomagnetic response during 11--13 November 2025. (a) Interplanetary magnetic field (IMF) magnitude $|B|$ and $B_z$ component (GSM coordinates). (b) Solar wind speed $V_\mathrm{sw}$ and dynamic pressure $P_\mathrm{dyn}$. (c) Interplanetary dawn-dusk convection electric field $E_y = -V_x \times B_z$ (GSM). (d) Geomagnetic indices $Dst$ and $AE$. The shaded region indicates the storm main phase window (UT 0--6 on 12 November 2025).
• Figure 2: Distribution of multi-source ionospheric observation networks in the China--Australia longitude sector (60--180$^\circ$E). Ground-based GNSS stations include CMONOC (China), IGS, GEONET (Japan), AGGA (Australia), and GDMS (Taiwan). Also shown are ionosonde stations and regional coverage areas.
• Figure 3: Global JPL GIM TEC and relative perturbation ($\Delta$TEC%, based on 27-day sliding IQR background) during 00--11 UT on 12 November 2025. For each UT snapshot, the upper panel shows TEC relative perturbation (%) with respect to the IQR-filtered median background, and the lower panel shows absolute TEC (TECU). Maps are in local-time coordinates, highlighting the rapid establishment of a dayside-dominant positive storm during the main phase. For complementary global perturbation maps during the remaining periods (11 November 00--23 UT and 12 November 12--23 UT), see Supporting Information Figures S1--S3.
• Figure 4: Sector-scale relative slant TEC change from BeiDou GEO observations. (a) Ionospheric pierce point (IPP) distribution for BeiDou GEO satellite links in the 60--180$^\circ$E sector; dashed lines indicate the latitude bins used for statistics. (b--f) Time series of $\Delta$STEC (TECU) for five latitude bands (30--50$^\circ$N, 15--30$^\circ$N, $-$15--15$^\circ$, $-$30--$-$15$^\circ$, $-$50--$-$30$^\circ$) on 12 November 2025 (UT). $\Delta$STEC is defined as a relative slant TEC change from an arc-start reference epoch (no explicit hardware-delay calibration) and therefore should not be interpreted as an absolute TEC value. The GEO geometry yields long slant paths, so $\Delta$STEC magnitudes can exceed typical VTEC levels; here the emphasis is on timing, morphology, and inter-latitude/hemisphere contrasts.
• Figure 5: Map-view evolution of GNSS-derived TEC disturbance (dTEC) in the 60--180$^\circ$E sector during the storm main phase. Panels show dTEC (TECU) at 20-min cadence from 01:00 to 06:00 UT on 12 November 2025. dTEC is obtained from GNSS TEC time series by arc segmentation, detrending with a Savitzky-Golay filter (order 2, 121-sample window), and a 4th-order zero-phase Butterworth low-pass filter retaining periods $>$40 min (see Methods). The sequence highlights coherent, traveling wave-like disturbance structures across the China--West Pacific--Australia region.