Sudden Stratospheric Warming Effects over L1 Scintillation at Low Latitude During Quiet and Magnetically Disturbed Periods

Abstract:

Small scale irregularities of hundred of meters, associated with bubbles cause scattering and diffraction of radio waves crossing the ionosphere and produces scintillation in amplitude and/or phase of the GNSS signal that can cause loss of lock of its code and/or carrier, affecting the positioning determination. The L1 band GPS amplitude scintillation intensities, represented by the S4 scintillation index, at the low latitude station of São José dos Campos (23.1º S, 45.8º W, dip latitude 17.3º S), located under the southern crest of the EIA, were analyzed during two northern hemisphere Sudden Stratospheric Warming (SSW) events. These events occurred during the northern winter months of 2003/2004 marked by moderate magnetic disturbances and 2012/2013 during a very quiet magnetic period. Normally during these months (January to February) moderate to strong scintillation occurs in this Brazilian station for moderate and high solar flux. Long lasting weakening of the scintillation amplitude at this low latitude station was observed during these two SSW events, compared to the pre-SSW periods, however stronger S4 weakening was observed during 2003/2004. The main mechanisms that can lead to scintillation weakening are the meridional neutral wind and the equatorial vertical plasma drift. Since no wind data is available during pre-SSW and SSW periods, we have sought to identify its signature in the latitudinal distribution of the TEC along the 60^o magnetic meridian and we suggest that a SSW induced southward meridional wind had a large contribution to the S4 weakening. The other mechanism that could have contributed to S4 weakening is the vertical plasma drift. This parameter, inferred from São Luís (2.52°S, 44.3°W, dip latitude 1.73°S) digisonde data for the 17 to 21 LT period during the SSW events, showed predominant decreases around the prereversal hours relative to their pre-SSW periods. The vertical drift during the period of the 2003/2004 SSW event presented a large flattening and remained constant at about 10 m/s. We suggest that this larger drift decrease during the magnetic storm, compared to the 2012/2013 SSW period, is caused by the SSW effects reinforced by the disturbance dynamo and overshielding westward polarity electric fields associated with the storm.