Three Second Waves Observed Upstream Of The Earth’s Bow Shock

A new class of ULF waves were discovered in the foreshock from the ISEE magnetometer data by Le et al. [1]. These unusual type of waves differ greatly from the more commonly observed 30 s waves, shocklets and SLAMS. The new waves have periods near 3 s and always show a very narrow spectrum that is in contrast with the lower frequency waves observed in the foreshock, which usually have broader spectra. Three second waves have large amplitudes and are observed in the upstream region only when the interplanetary magnetic field intersects the bow shock and when the plasma beta is high. Three second waves can be divided in three types. Isolated waves are associated with reflected cold beams and are observed in regions where the magnetic field is very quiet. Superposed waves are associated with more intermediate ion distributions, and are superposed on non steepened lower frequency waves. Irregular waves are associated with diffuse ions and are observed as trains surrounded by an irregular magnetic field. Three second waves are generated by the right‐hand non resonant instability, being right‐handed and propagating downstream in the plasma frame. This instability grows due to the interaction of the solar wind distribution with a reflected ion beam. In this work we map the location of the different types of three second waves with respect to the bow shock, and study their evolution in the foreshock. We also study which instability is generating the lower frequency waves observed with the superposed 3 s waves.