Geodetic and Atmospheric Background - Atmospheric Effects in Space Geodesy - page 28

The low latitude region contains the highest values of the Total Electron Content
(TEC) and the peak electron density. The scintillation effect (see Sect. 4.3.3 ) has its
greatest amplitude at these latitudes. The distribution of the peak electron density at
the F2 layer depicts a minimum at the geomagnetic equator with twomaximum peaks
on both sides of the equator, at themagnetic latitudes of 15 ◦ -20 ◦ north and south. This
phenomenon is called equatorial (or Appelton) anomaly and this region is also called
the equatorial region. Investigations by several authors (e.g. Hoque and Jakowski
( 2012 )) show that the peak over the geomagnetic equator extends during daytime,
but becomes weaker during nighttime. Various processes significantly disturb these
areas, which display a strong diurnal dependence.
The mid latitude region is the least variable region of the ionosphere. It shows the
most regular and predictable variations of TEC. There are several ionospheric models
that estimate the mean ionosphere in this region with a high degree of accuracy.
Nevertheless, the daily variations of TEC in this region reveal a root mean square
(r.m.s.) variations of 20-30% from the average value. Within this region there are
zones of low electron densities lying between 50 ◦ -70 ◦ geomagnetic latitude called
the mid latitude troughs (Muldrew 1965 ). The electron density inside the trough is
drastically reduced by as much as a factor of 2 at 1000km altitude and as much as
an order of magnitude at the F2 peak (Timleck and Nelms 1969 ).
In the high latitude region photo-ionization is the main source of ionization.
Another important driver in this region is the high energy particles. Geomagnetic
field lines guide energetic protons and electrons from the magnetosphere down to
the Earth's atmosphere. Accelerating particles lose their energy after colliding with
the neutral particles and ionize them at the same altitude where solar UV radiation is
absorbed by the atmosphere. Precipitating particles also lose their energy before col-
lisions with the neutral particles through the particle-wave interaction, which finally
generates intense electromagnetic waves named auroral kilometric radiation (AKR)
with a frequency between 100 and 500kHz. Additionally, some of the atmospheric
elements are excited to higher energy levels. This leads to emission of visible lights,
called the auroral lights. This activity occurs mainly within the auroral oval. The
maximum is near 67 ◦ N at midnight, increasing to about 77 ◦ N at noon. They tend to
occur in bursts, each lasting about 30-60min, separated by intervals of several hours.
4.3.3 Regular Temporal Variations
Variations within the solar radiation and the solar zenith angle causes temporal varia-
tions of the ionosphere. These variations could be classified into regular and irregular
variations. The ionosphere exhibits daily, seasonal and longer variations controlled by
the solar activity. Over the course of the day, season, and sunspot cycle the ionosphere
parameters might change by several orders of magnitude. However, during irregular
variations, the change can happen within a few minutes up to several days.
The solar cycle variations are long-period in the ionosphere depending on the
solar activity. A basic indicator for the level of solar activity is the sunspot number.
Due to their relatively low temperature, the sunspots are visible on the solar surface