JGR:Space physics

High-precision measurements of the magnetic field are critical to explore the near-Mars environment. The Mars Orbiter MAGnetometer (MOMAG) is one of seven payloads on board the Tianwen-1 orbiter. Its zero offset needs regular calibration, and the Wang-Pan method I is applied to the MOMAG when the Tianwen-1 orbiter is in the solar wind. The orbiter will remain out of the solar wind over tens of days each year, a method is necessary to ensure the accuracy of the zero offset during this period. Recently, a new method was proposed by Wang (2022b), https://doi.org/10.3847/1538-4357/ac822c, which is referred to as the Wang method II for ease of description. Here, we test the performance of this method using the MOMAG data measured in the Martian magnetosheath. We find that the zero offset O w determined by the Wang method II varies around the zero offset O wp calculated by the Wang-Pan method I using the potential Alfvénic fluctuations in the solar wind. After smoothing with a temporal window of 27 days, O w is able to achieve an accuracy close to O wp. If the data segment has no gaps and its duration is < 27 days, the smoothed O w might have an error >2 nT, but the error tends to be smaller if the segment's duration is longer. Our tests suggest that the Wang method II is suitable for the in-flight calibration of the MOMAG when the Tianwen-1 orbiter remains out of the solar wind.

Observations from the Michelson Interferometer for Global High-Resolution Thermospheric Imaging onboard the Ionospheric Connection Explorer spacecraft are used to study the response of OI630.0 and OI557.7 nm dayglow to a moderate geomagnetic storm on 27 August 2021. The storm reaches a minimum Dst index of −82 nT, significantly impacting the dayglow within the latitudinal range of approximately 20°N–42°N, where the dayglow observations are of good quality. During the geomagnetic storm, the OI630.0 dayglow intensity slightly increases, while the peak volume emission rate (VER) decreases, and the peak height rises noticeably. The F-layer intensity, peak VER, and the entire-layer intensity of OI557.7 dayglow decrease significantly. The rise in peak height is not noticeable for the OI557.7 dayglow. The VERs of the both dayglow emissions respond differently to the geomagnetic storm at different altitudes. The OI630.0 dayglow layer as a whole extends upward and rises in altitude. For dayglow averaged above 35°N, the OI630.0 dayglow VER increases above approximately 225 km but decreases below this altitude. The largest increase occurs near 300 km, reaching approximately 82.8%, while the largest decrease occurs around 160 km, reaching about −22.0%. The OI630.0 dayglow intensity increases by approximately 6.3%, the peak VER decreases by about −8.0%, and the peak height rises by approximately 16.3 km, corresponding to a 7.8% increase. The F-layer intensity, peak VER, and the entire-layer intensity of OI557.7 dayglow decrease by approximately −27.5%, −32.4% and −17.4%, respectively. The response of the dayglow also depends on longitude and is accompanied by a southward meridional wind.

The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and CHAllenging Minisatellite Payload (CHAMP) satellites measure in-situ thermospheric density and cross-track wind. When propagating obliquely to the satellite track in a horizontal plane (i.e., not purely along-track or cross-track), gravity waves (GWs) can be observed both in the density and cross-track wind perturbations. We employ the Wavelet Analysis, red noise model, dissipative dispersion and polarization relations for thermospheric GWs, and specific criteria to determine whether a quiet-time (Kp < 3) thermospheric traveling atmospheric disturbances (TADs) event is a GW or not. The first global morphology of thermospheric GWs instead of TADs is reported. The fast intrinsic horizontal phase speed (c IH > 600 m/s) of most GWs suggests that they are not generated in the lower/middle atmosphere (where c IH < 300 m/s). A second population of GWs with slower speeds (c IH = 50–250 m/s) in GOCE are likely from the lower/middle atmosphere, but they occur much less frequently in CHAMP. GW hotspots occur during the high-latitude and the winter midlatitude regions. GW amplitudes exhibit semi-annual and annual variations. These findings suggest that most GOCE and CHAMP GWs are higher-order GWs from primary GW sources in the lower/middle atmosphere. Finally, the average propagation direction of the CHAMP GWs exhibits a clear diurnal cycle, with clockwise (counterclockwise) occurring in the northern (southern) hemisphere and equatorward propagation occurring at ∼13 LST. This suggests that the predominant GW propagation direction is opposite to the background wind direction.