  The location of the terrestrial magnetopause (MP) and it's subsolar stand-off
distance depends not only on the solar wind dynamic pressure and the
interplanetary magnetic field (IMF), both of which play a crucial role in
determining it's shape, but also on the nature of the processes involved in the
interaction between the solar wind and the magnetosphere. The stand-off
distance of the earth's MP and bow shock (BS) also define the extent of
terrestrial magnetic fields into near-earth space on the sunward side and have
important consequences for space weather. However, asymmetries due to the
direction of the IMF are hard to account for, making it nearly impossible to
favour any specific model over the other in estimating the extent of the MP or
BS. Thus, both numerical and empirical models have been used and compared to
estimate the BS and MP stand-off distances as well as the MP shape, in the
period Jan. 1975-Dec. 2016, covering solar cycles 21-24. The computed MP and BS
stand-off distances have been found to be increasing steadily over the past two
decades, since ~1995, spanning solar cycles 23 and 24. The increasing trend is
consistent with earlier reported studies of a long term and steady decline in
solar polar magnetic fields and solar wind micro-turbulence levels. The present
study, thus, highlights the response of the terrestrial magnetosphere to the
long term global changes in both solar and solar wind activity, through a
detailed study of the extent and shape of the terrestrial MP and BS over the
past four solar cycles, a period spanning the last four decades.
