LINDA - Langmuir Interferometer and Density experiment for Astrid-2LINDA is our contribution at the Uppsala Division to Astrid-2, for which the principal investigator is Bengt Holback. Our main scientific aim is the measurement of density fluctuations in the auroral plasma, and also studies of the background plasma density. By using two spherical Langmuir probes (1 cm in diameter) interferometric measurements are possible. The distance between the probes is 2.9 meters, so by measuring the time delay between detection at the two probes, the motion of plasma structures up to velocities of 23 km/s (about three times the satellite speed) can be resolved.
The Langmuir probes of LINDA are sampled at three different frequencies. Measurements at 16 samples/s, 16 bits resolution, gives information on variations of plasma density (n) such as the structure of the auroral plasma. The n-data are sampled continuously during a measurement. Measurements at 8192 samples/s, 8, 10 or 12 bits resolution, gives information on plasma wave phenomena (dn/n). The dn/n-data are sampled in variable length snapshots during a measurement. In addition to these measurements does LINDA perform diagnostic so called Langmuir sweeps at regular intervals (1 or 2 minutes). The voltage bias on the probes are then stepped so that the probe characteristics can be obtained (Current vs. Voltage). From the Langmuir sweeps are plasma characteristics such as electron temperature and absolute density derived. The sampling rate at a sweep is 512 samples/s, 16 bits resolution. A measurement by LINDA comprises all three types of data described above. The digital part of the LINDA instrument has been designed, built, and programmed by Åke Jacksén as an engineering diploma work.
LINDA uses a new boom concept, developed and manufactured by Bengt Holback and Harley Thomas for low weight, low cost, and high reliability. During launch are the LINDA booms folded up around the folded solar panels. Once in orbit, the solar panels are unfold, and in this process are the stiff booms automatically released. The deployment scheme is completely mechanical, without any electrical or pyrotechnical devices. By this mounting are we able to get a relatively long probe-to-probe distance (2.9 m) using rather short (0.6 m) booms. The mounting on the solar panels is possible only by the very low weight of the LINDA booms at about 100 g each, including probes, hinges, and locks.