# Equatorial Plasma Bubbles: A Review
## 2 Observations of EPBs and Associated Irregularities
### Magnetic Signatures of EPBs  
Some satellites equipped with magnetometers have also allowed for simultaneous measurements of fluctuations in electric fields and magnetic fields associated with EPBs, which has revealed some new features of EPBs and suggests alternate theories for EPBs, which shall be briefly discussed in Section 3. Instruments on board NASA's Dynamic Explorer 2 (DE 2) satellite measured electric and magnetic field fluctuations associated with EPBs updrafting at supersonic speeds, which led [PERSON] et al. [26] to suggest that Alfven waves are launched in the equatorial F region when an EPB starts to grow there and that these waves carry geomagnetic field-aligned currents, which couple the equatorial F region with conjugate E regions at the feet of the geomagnetic field. Coupling of the equatorial F region with conjugate E regions through geomagnetic field lines is the reason that EPBs are a post-sunset phenomenon, as discussed in Section 3. Electric and magnetic field sensors of the Extremely Low Frequency Wave Analyzer instrument on board the Combined Release and Radiation Effects Satellite (CRRES) detected both electrostatic waves with a small magnetic field component and electromagnetic waves propagating in the extraordinary mode within EPBs mostly in the altitude range of 400-500 km [27]. The French microsatellite mission DEMETER (Detection of Electromagnetic Emissions from Earthquake Regions) also detected electromagnetic fluctuations associated with EPBs in regions with plasma density structures of scale sizes 1-10 km, whereas broadband electrostatic fluctuations were observed in regions with structures of 10-100 m in scale [28].  
Magnetic field fluctuations associated with EPBs have been studied extensively using data from the high-resolution vector magnetometer on board the German CHAMP (Challenging Minisatellite Payload) satellite launched in 2000 [29; 30]. Vector magnetic field measurements provided estimates of fluctuations in the component of the magnetic field parallel to the main geomagnetic field, as well as fluctuations in the components transverse to the main geomagnetic field. The latter could be produced by field-aligned currents. The CHAMP satellite's orbit was circular and nearly polar, with a high inclination (87.3\({}^{\circ}\)) and initial altitude of about 450 km. In situ plasma density measurements made by a planar Langmuir probe (PLP) showed the presence of EPBs at the height of the satellite. An example of the magnetic field fluctuations associated with EPBs that were present around 38\({}^{\circ}\) W geographic longitude is shown in Figure 1. On this day, fluctuations with a maximum amplitude of \(\pm\)4 nT are seen in the magnetic field components transverse to the mean geomagnetic field in the region with the deepest depletions in plasma density. Swarm, ESA's mission for Earth observation with a constellation of three identical satellites, was launched in 2013. The satellites' orbits are nearly polar, and they are equipped with vector field magnetometers, Langmuir probes, and thermal ion imagers to measure ion drift, from which electric fields could be estimated [31; 32; 33]. These observations have been used to study, among other aspects of EPBs, the geomagnetically conjugate characteristics of EPB irregularities [32].