Relationship Between Low‐Latitude Pi2 Pulsations and Cavity Mode Oscillations

• Published in: Journal of geophysical research. Space physics Vol. 129; no. 9
• Main Authors: Takahashi, Kazue, Lysak, Robert, Majmudar, Khilav
• Format: Journal Article
• Language: English
• Published: 01.09.2024
• Subjects: cavity mode oscillations; global propagation; ground magnetometersm; low latitudes; Pi2 pulsations; Van Allen Probles
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2024JA032858

On the ground, Pi2 magnetic pulsations are detected at low latitudes (L<2) $(L< 2)$ at all magnetic local times (MLTs), unlike in the inner magnetosphere. To gain insight into the mechanism for the global appearance, we study the MLT dependence of the properties of low‐latitude ground Pi2 pulsations detected at four longitudinally separated stations. The pulsation properties are defined with respect to compressional magnetic field Bμ $\left({B}_{\mu }\right)$ oscillations detected by Van Allen Probes at L $L$ = 2.5–6.5 within 2 hr of midnight. Up to two peaks between 6.7 and 40 mHz found in the Bμ ${B}_{\mu }$ spectrum are selected as possible signatures of the source of ground Pi2 pulsations. For each spectral peak, we compute the coherence of the ground horizontal northward (H) $(H)$ component with Bμ ${B}_{\mu }$, and those events exhibiting high coherence are used in statistical analyses. The radial mode structure of the Bμ ${B}_{\mu }$ oscillations indicates they are fundamental or second harmonics of cavity mode oscillations (CMOs). Ground pulsations appear primarily in the H $H$ component with time delays of less than a few seconds and amplitudes comparable relative to the Bμ ${B}_{\mu }$ oscillations in the low‐L $L$ region. The observations suggest that, if the dayside ground Pi2 pulsations are driven by ionospheric currents as previously proposed, the current must be coupled to the CMOs, not to the currents flowing on field lines connected to the auroral zone. Plain Language Summary Ground magnetometers routinely detect oscillations called Pi2 pulsations (periods of around 1 min) after sudden changes of the nightside magnetosphere called substorms. The ground pulsations are detected at all local times including noon if observations are made at low latitudes. Despite many observational and theoretical studies, questions still remain regarding how the oscillations establish the periodicity and propagate from the nightside to the dayside. To find the answers, we statistically study the magnetospheric properties of the oscillations and how they are related to low‐latitude Pi2 pulsations on the ground, using combined data from the Van Allen Probes spacecraft and ground magnetometers located at four well‐separated longitudes. We confirm that magnetospheric oscillations related to ground Pi2 pulsations are cavity mode oscillations (CMOs). On the ground, dayside pulsations are detected with time delays of a few seconds and comparable amplitudes with respect to pulsations at midnight. Ionospheric currents connected to the CMOs are a possible source of the dayside Pi2 pulsations. Key Points Dayside ground Pi2 pulsations exhibit little time delay from magnetospheric compressional oscillations The magnetospheric oscillations are cavity mode oscillations excited at multiple harmonics If ionospheric currents produce the ground Pi2 pulsations, the currents are coupled to the cavity mode