Observations of Mesospheric Gravity Waves Generated by Geomagnetic Activity

Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 and 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55 degrees latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is often seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region. Gravity waves (GWs) exist throughout the atmosphere and are crucial in the dynamics of the middle and upper atmosphere. A variety of processes are known to excite GWs at different altitudes. Above 100 km, space weather induced geomagnetic activity is an important source for the GWs. However, how deep such waves penetrate into the mesosphere, and in what latitude regions their effect is important remains unknown. In this work, we use SABER/TIMED satellite measurements of temperature between 2002 and 2018 to investigate this question. For the first time, we find that the geomagnetic activity forces mesospheric GWs only in the high latitude regions, where enhanced energy deposition occurs along magnetic field lines. Further, these GWs occur only above 80 km, and no unambiguous signature is seen at lower heights. Though damping is expected due to the increasing atmospheric density, this work identifies the altitude and latitude extent for such GWs forced by geomagnetic activity in the mesosphere. Further, there is a significant seasonality in the response such that summer hemisphere shows weakest GW generation due to geomagnetic activity. The mesopause height is also observed to descend sometimes during intense geomagnetic disturbances occurring in non-summer periods. Geomagnetically forced gravity waves penetrate down to similar to 80 km only in the high latitude regions as revealed by Sounding of the Atmosphere using Broadband Emission Radiometry temperature data Summer high latitude mesosphere is less responsive for gravity wave generation due to geomagnetic activity Significant variability in the gravity wave response is noticed even between severe geomagnetic disturbances occurring in the same season