Tuesday, 24 May 2011

Solar Activity Dependence of Ion Upflow Observed with EISCAT UHF Radar

Today our Science Tuesday post deals with the phenomenon of ion upflow in the polar ionosphere, where the geomagnetic field lines are very steep, almost vertical to the surface of the Earth. Charged particles travel along these lines, and given the right conditions, even heavy ions flow upwards, i.e. away from Earth.

The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromsø UHF radar between 1984 and 2008. In agreement with other work we find that the upward ion flux is generally higher at high solar activity than at low solar activity. However, the frequency of ion upflow events is higher at low solar activity and also the upward velocity is higher at low solar activity.

In any year about 30–40% of ion upflow is accompanied by ≈500 K above-background electron temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger at low solar activity than at high solar activity. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. While ions start to flow up at above 450 km altitude when solar activity is high, at low solar activity this happens at 300–500 km altitude. It is suggested that the solar activity influences long-term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2-density peak, and ion-neutral collision frequencies in the thermosphere and ionosphere.

Figure: (a) Occurrence frequency of ion upflow starting between 200 and 550 km altitude at nighttime between 1900 and 0500 MLT is shown between 1984 and 2008: 1 year average (black line) and 3 month average (gray circles). (b) Average field-aligned ion velocity at 400 km altitude. Red circles: average flux in upflow events; blue circles: average flux of all samples (upflow, no-flow, and downflow events). (c) Average field-aligned flux at 400 km altitude; circles every 3 months.

Original article: Y. Ogawa, S. C. Buchert, A. Sakurai, S. Nozawa, and R. Fujii, Solar activity dependence of ion upflow in the polar ionosphere observed with the EISCAT Tromsø UHF radar, J. Geophys. Res., 115, A07310, doi:10.1029/2009JA014766, 2010.

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