Orbit

THOR orbit
The key science regions of the THOR mission are the magnetosheath, the bowshock, the foreshock and the pristine solar wind. For an experimentally successful mission, THOR should return high quality, high cadence data from at least:
  • 50 quasi-parallel shock crossings
  • 20 quasi-perpendicular shock crossings
  • 100 h slow solar wind
  • 50 h fast solar wind
  • 50 h foreshock
  • 100 h magnetosheath behind quasi-parallel shock 
  • 20 h magnetosheath behind quasi-perpendicular shock
Each of the different regions will need to be sampled under a variety of conditions to study the variability, and therefore understand the physics of the different forms of plasma energisation. The dwell time of THOR in each regions thus has to exceed the times listed above. To target the four key science regions and fulfil the mission criteria listed above, the mission is divided into three phases each with a duration of at least one year and with perigee at 6 RE but with different apogees to specifically target the different regions:
  1. Magnetosheath and bowshock: 15 RE apogee, period 2 days
  2. Forsheock and pristine solar wind: 26 RE apogee, period 3.75 days
  3. Pristine solar wind: perigee: 45 RE apogee, period 7.5 days

Bowshock crossings

The choice of apogee is most important during phase one, which targets the bowshock and the magnetosheath. The location of the bowshock and magnetopause are dependent on the solar wind conditions, which are highly variable. It is therefore the motion of the bowshock that results in most of the observed bowshock crossings, and not the motion of the spacecraft. For the solar conditions expected during the launch of THOR, the bowshock nose distance is expected to be at around 13 RE but also extending beyond 15 RE. To spend a large amount of time both at the bowshock but also behind it in the magnetosheath, the apogee during phase one is set to 15 REBased on expected solar wind conditions, a large number of both quasi-parallel and quasi-perpendicular bowshock crossings will be observed.