Extreme O$$^{+}$$ enrichment in the deep inner magnetosphere: the May 2024 geomagnetic storm

Abstract During the super geomagnetic storm of 10–11 May 2024, an extreme enhancement in plasma mass density was observed in the deep inner magnetosphere near $$L \sim 2.5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>L</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:math> . Multi-point ground magnetometer observations revealed that this enhancement extended across widely separated longitudinal sectors—from New Zealand through Europe to eastern North America—during the storm main phase and early recovery phase. The maximum density, approximately 35,000 amu/cm $$^{3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>3</mml:mn> </mml:mmultiscripts> </mml:math> , was detected near $$L = 2.1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>L</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2.1</mml:mn> </mml:mrow> </mml:math> in the New Zealand longitude sector during the storm main phase. To investigate the origin of this anomalous mass loading and the associated highly O $$^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>+</mml:mo> </mml:mmultiscripts> </mml:math> -rich plasma state, we employ an integrated analysis combining multi-point ground magnetometer measurements, Arase satellite observations, DMSP satellite data, and total electron content (TEC) distributions derived from global GNSS networks. Ground-based magnetometer observations provide spatially distributed field line resonance (FLR) signatures that enable estimation of equatorial plasma mass density based on assumed field-aligned density profiles. Arase in situ measurements of plasma wave spectra, magnetic fields, and energetic particle fluxes enable estimation of local plasma density and characterization of ion and electron energy distributions. DMSP-F17 observations supply complementary ionospheric parameters including electron temperature, while GNSS TEC maps reveal large-scale ionospheric electron depletion and its regional evolution. This coordinated multi-dataset approach enables systematic characterization of the unique inner magnetospheric plasma state during this extreme event. Plasmaspheric electron densities derived from Arase plasma wave measurements indicate in situ electron densities of approximately 1,500 cm $$^{-3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:math> at $$L \sim 2.5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>L</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:math> . Combined with mass density estimates, the inferred ion composition consistently indicates heavy-ion dominance, with O $$^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>+</mml:mo> </mml:mmultiscripts> </mml:math> fractions exceeding 90% in some regions. The coexistence of cold and warm plasma populations observed by Arase near the plasmapause, together with elevated ionospheric electron temperatures detected by DMSP-F17 and significant TEC depletion, suggests that cold plasmaspheric electrons were heated through Coulomb collisions with storm-time injected warm ions. This process likely led to enhanced heating of ionospheric electrons and subsequent heavy-ion upflow along affected flux tubes. These results indicate that superstorm-level magnetospheric convection can produce rapid plasma mass loading at unusually low L -shells during the storm main phase, leading to the formation of an O $$^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>+</mml:mo> </mml:mmultiscripts> </mml:math> -rich plasmasphere, in contrast to the conventional recovery-phase refilling scenario. The findings highlight the critical role of ionospheric outflow in regulating inner magnetospheric plasma mass density under superstorm conditions.