3.3 The Features Related to Geomagnetic Activity
at High Latitudes
It is well known that ionospheric irregularities at high
latitudes are mostly caused by plasma processes associated
with auroral activities. These activities are attributed to
particle precipitation and dynamical processes including high
speed plasma convection, which make the ionospheric density
structure highly irregular. Yet, plasma instabilities and
gravity waves, launched in the thermosphere due to heating
processes, contribute to various spatial scales of ionospheric
irregularities. All these processes are greatly enhanced during
geomagnetic storms and thus under such conditions affect the
propagation of trans-ionospheric radio signals. Figure 4
shows an example of GPS phase fluctuations as measured by
ROTI on the same day as shown in Figure 3, but in a northern
auroral region (Yellowknife, Canada, 62.5øN, 245.5øE, dip
latitude = 73.2øN). Strong GPS phase fluctuations (maximum
ROTI -- 5) were observed at this station throughout 24 hours in
almost entire region.