EXPERIMENTAL-EVIDENCE FOR THE FORMATION AND ENTRY OF PATCHES INTO THEPOLAR-CAP

A mechanism for the formation of polar cap patches is proposed based o n data collected by the Sondrestrom incoherent scatter radar, magnetom eters located on the west and east coasts of Greenland and Digisondes, digital ionospheric sounders, operated at Qaanaaq and Sondrestrom. Th is instrumentation has served to identify the formation of patchlike s tructures and follow their temporal evolution and entry into the polar cap. Prior to the onset of the event, the background ionospheric plas ma was moving in a poleward direction and formed part of a tongue of i onization directed toward and into the polar cap. The event starts wit h the appearance of a fast plasma jet containing eastward directed vel ocities in excess of 2 km s-1. This plasma jet consists of a channel e xtending 300 km in width, where the F region ion temperature reaches v alues in excess of 4000 K and the E layer T(i) is enhanced above 5000 K. The existence of a region containing high electron temperatures ass ociated with soft precipitation (likely the cusp) and located adjacent and equatorward of the fast plasma jet is very suggestive of the jet being the result of a tension force acting upon recently reconnected f ield lines. This view is supported by the fact that the fast plasma je t was seen in the prenoon sector under IMF B(y) negative conditions. T he elevated T(i) values inside the plasma jet are exactly collocated w ith depleted F region densities. We suggest that the recombination los s of O+ is increased by a factor >10 due to the dependence of the O+ N2 reaction on the ion temperature. The eroding action of the fast pl asma jet proceeds until the poleward moving tongue of ionization is di vided into regions containing high and low densities. Magnetic field p erturbations associated with the fast plasma jet were observed by the different magnetometer stations. The large negative bays recorded at m ost of the sites have been used to trace the poleward motion of the je t. At the time that the fast plasma jet reaches Qaanaaq, the Digisonde there measured low f(o)F2 values. A few minutes before and after this minimum, high f(o)F2 Values were observed. The ratio of the enhanced f(o)F2 Values on either side of the minimum, to the minimum f(o)F2 is about 2 (or a density ratio of 4). This is the commonly accepted signa ture of a polar cap patch. The series of events leading to the formati on of the patchlike density structure has been designated a density br eakoff event.