Longitudinal Variation of the Lunar Tide in the Equatorial Electrojet

Abstract

The atmospheric lunar tide is one known source of ionospheric variability. The subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere. There is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude. We use magnetic field measurements from the CHAMP satellite during July 2000 to September 2010 and from the two Swarm satellites during November 2013 to February 2017 to determine, for the first time, the complete seasonal‐longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity. Significant longitudinal variability is found in the amplitude of the lunar tidal variation, while the longitudinal variability in the phase is small. The amplitude peaks in the Peruvian sector (∼285°E) during the Northern Hemisphere winter and equinoxes, and in the Brazilian sector (∼325°E) during the Northern Hemisphere summer. There are also local amplitude maxima at ∼55°E and ∼120°E. The longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field. Another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides. A tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward propagating mode with zonal wave number 2 (SW2), with secondary contributions by westward propagating modes with zonal wave numbers 3 (SW3) and 4 (SW4). Eastward propagating waves are largely absent from the tidal spectrum. Further study will be required for the relative importance of ionospheric conductivities and nonmigrating lunar tides.

The Comparison of Lunar Tidal Characteristics in the Low‐Latitudinal Ionosphere Between East Asian and American Sectors During Stratospheric Sudden Warming Events: 2009–2018

“…Qualitatively, the clarity of a TS signature indicates the relative importance of the TS-generating process compared to other driving factors. Yamazaki et al (2017) find significant longitudinal variability in the amplitude of the semidiurnal lunar variation in the EEJ intensity derived from CHAMP and Swarm observations. To be specific, a larger RS M2 corresponds to a clearer TS pattern.…”

September 2019 Antarctic Sudden Stratospheric Warming: Quasi‐6‐Day Wave Burst and Ionospheric Effects

“…The semidiurnal lunar variations in the EEJ intensity derived from the Swarm A and B data during 5 September to 5 October 2019 are presented in the supporting information (Figure S4). It is found that the amplitude of the EEJ semidiurnal lunar variation is 17.7 2.1 mA/m for Swarm A (14:00–17:00 MLT) and 16.6 2.8 mA/m for Swarm B (11:00–14:00 MLT), which is greater than the climatological value of 9.0 0.4 mA/m as reported by Yamazaki et al () for September daytime (08:00–16:00 local solar time) conditions. The phase, which is defined as the lunar time of maximum, is 10.2 0.2 hr for Swarm A and 10.0 0.4 hr for Swarm B, which is in good agreement with the climatological value of 10.0 0.1 hr.…”

Lunar Tide in the F Region Ionosphere

“…From this perspective the present results are qualitatively consistent with the wavenumber broadening influence of Earth's magnetic field as noted by Yue et al () in the context of the quasi‐2‐day wave. Yamazaki et al () found the major nonmigrating components of M 2 in the EEJ to be s =3, 4 with respect to geographic longitude. Both of these results stand in contrast to TIMED/SABER temperature analyses (Paulino et al, ) and modeling that includes ocean and Earth tide forcing (Vial & Forbes, ), wherein the M 2 tide in the neutral atmosphere is described by zonal wavenumbers s =−3 to +5.…”

“…The nonmigrating tides account for zonal asymmetries in the lunar tide. Apart from the dominant SW2, Yamazaki et al () found the major nonmigrating components of M 2 in the EEJ to be SW3 and SW4. Eastward propagating components were not found to play a role in defining the longitude dependence of M 2 in the EEJ.…”

“…CHAMP Planar Langmuir Probe measurements have in fact been analyzed to reveal seasonal‐latitudinal and longitude variability in electron density and interpreted in the context of nonmigrating solar tides (Xiong & Lühr, ; ; Xiong et al, ; Zhou et al, ). Magnetic field measurements from CHAMP have also revealed significant longitude variability in the midlatitude solar‐quiet (Sq) current system (Pedatella et al, ) and in the EEJ (Lühr et al, ; Lühr, Siddiqui, & Maus, ; Lühr, Rother et al, ; Yamazaki et al, ). Lühr, Siddiqui, and Maus () in fact provide the first global study on the lunar tidal modulation of the EEJ.…”