The extended solar minimum conditions in 2008 and 2009 presented an opportunity to investigate the ionosphere at lower solar activity levels than previously observed. The Coupled Ion Neutral Dynamics Investigation (CINDI) Ion Velocity Meter (IVM) instrument onboard the Communication/Navigation Outage Forecasting System is used to construct the median meridional (vertical) ion drifts, ion densities, and O+ concentrations during periods of low geomagnetic activity for four characteristic seasons each year spanning late 2008 to 2010. The presence of a large semidiurnal component in the ion drift variation at the equator produced significant differences from typical ionospheric conditions. Instead of upward drifts during the day and downward drifts at night, downward drifts in the afternoon and upward drifts near midnight are observed. This semidiurnal component is present in all seasons though it is strongest during the solstice seasons. It is shown that upward drifts at night correspond to regions with a high occurrence of postmidnight irregularities during the December 2008 and June 2009 solstices. A comparison with vertical ion drifts observed by the Jicamarca Radio Observatory supports the methodology used to extract meridional drifts from the IVM.

[1] While the mechanism for producing plasma irregularities in the dusk sector is believed to be fairly well understood, the cause of the formation of irregularities and bubbles during the postmidnight sector is still unknown, especially for magnetically quiet periods. This paper presents a case study of the strong postmidnight bubbles that often occur during magnetically quiet periods primarily in June solstice, along with a 4 year (2009)(2010)(2011)(2012) statistical study that shows strong occurrence peak during June solstice predominantly in the African sector. We also confirm, for the first time, the presence of Rayleigh-Taylor (RT) instability during postmidnight hours by using the physics-based model for plasma densities and RT growth rates. Finally, we consider several possible sources of the eastward electric fields that permit the RT instability to develop and form bubbles in the postmidnight local time sector. Citation:

[1] Local-time variations of zonal drifts at different latitudes, longitudes, and seasons are investigated using ion drift measurements from the Republic of China Satellite-1 (ROCSAT-1). We select a quiet time period defined when Dst > −100 nT and K p ≤ 3. The ion drifts in the magnetic zonal direction are latitude (apex height) dependent and differ for different longitude sectors and seasons. Superrotation of the ionosphere is observed in the zonal drift measurements, particularly at the lower latitudes. The ionospheric superrotation generally decreases with increasing latitude and is attributed mainly to the F region dynamo where the features of the neutral wind and conductance affect the characteristics of the superrotation at different latitudes, longitudes, and seasons. We suggest that the latitudinal variation of the superrotation is mainly produced by the decrease in latitude of the neutral winds in the F region. A decrease of conductance with increasing latitude is also present, but evidence shows that neutral winds decrease faster with latitude than the conductance for high solar flux conditions. The longitudinal and seasonal variations of the superrotation are more influenced by changes in the F region conductance that is modified by vertical E × B ion drift motions and neutral wind-induced ion drift motions along magnetic field lines.

[1] In situ ion drift observations from the ROCSAT-1 satellite, near the 600 km altitude and for the years [1999][2000][2001][2002][2003], are used to describe variations in the local time (LT) distribution of meridional E × B drifts at low and middle latitudes, for different longitudes and seasons. We derive the ion drifts in the direction perpendicular to the magnetic field during quiet times defined by Kp ≤ 3 and Dst > −100 nT. Offsets in the original data are removed by considering separately the northbound and southbound passes in a given volume and by enforcing magnetic conjugacy for the ion drift components perpendicular to the magnetic field. Three major features are identified for which the variations in latitude, longitude, and season allow the major drivers of these features to be indicated: (1) a latitude/magnetic apex height variation in the prereversal (after sunset) enhancement of the vertical drift that is attributed to similar variations in the flux tube-integrated Pedersen conductivity, (2) the daytime meridional drifts that are largely driven by the E region dynamo show significant latitude variations that are attributed to the relative importance of diurnal and semidiurnal wind fields as a function of latitude, and (3) a downward enhancement of the ion drift before sunrise is attributed to a reversal in the F region zonal wind during the nighttime.