# Equatorial Plasma Bubbles: A Review
## 2 Observations of EPBs and Associated Irregularities
### In Situ Observations Using Rockets and Satellites  
In situ plasma density and electric field measurements made by instruments flown on rockets and satellites provide information on variations in plasma density and electric fields over a large range of scale sizes and have therefore been used extensively for spectral studies of the ionospheric irregularities associated with EPBs [14; 15; 16; 17; 18; 19]. With satellite observations, information is obtained about variations essentially in a horizontal direction, while rocket flights provide information about variations in altitude. Such studies have shown that the one-dimensional spectrum of plasma density variations associated with the ESF irregularities, measured by instruments on board rockets or satellites, has a power-law form. One of the first efforts to study the EPB phenomenon, using plasma density probes on board a rocket, was a multi-instrument investigation involving simultaneous rocket and 50 MHz radar observations in Natal, Brazil [20]. In this experiment, a sounding rocket was launched during ESF conditions at nighttime. Plasma density probes on board detected strong fluctuations in plasma density below the F layer peak, and a large bite out in plasma density just below the peak. This was followed by optical observations of a barium cloud released from the rocket, and radar observations of a weak patch of irregularities moving radially away. The combined observations led the authors to suggest that EPBs move upwards after their generation in the bottom side of the post-sunset equatorial F region [20]. In situ measurements of plasma density using probes on board rockets and satellites, in conjunction with radar observations of backscattered radio signals, have provided the connection between plumes observed in radar RTI maps and EPBs. In an early campaign, a rocket carrying high-resolution plasma density probes, electric field probes, and other instruments was launched so that it would penetrate a well-developed plume above the equatorial F region peak, as seen in the RTI maps obtained from the ALTAIR radar [21]. The rocket data provided an altitude profile of plasma density variations associated with the present irregularities and their spectral characteristics in the vertical direction. In another coordinated study, the east-west profile of ion density variations within EPBs, measured in situ at an altitude of about 373 km by an ion drift meter on board the Atmospheric Explorer E (AE-E) satellite, was compared with the backscatter plume obtained from the ALTAIR radar [22]. The authors concluded that, in the plane perpendicular to the geomagnetic field, EPBs are vertically elongated, tilted wedges that extend from the bottom side of the equatorial F layer to the top side. Data collected by polar-orbiting satellites of the Defense Meteorological Satellite Program (DMSP) from 1989 to 2002 showed how the occurrence rate of EPBs at the height of the satellites (~840 km) varied with longitude and day of the year, which had an important connection with longitudinal variation of the declination of the main geomagnetic field at the dip equator [23]. Considering the major disruptive role that irregularities that develop within EPBs may play in the operation of satellite-based communication and navigation systems, in 2008 a satellite was launched by the United States Air Force to specifically study this phenomenon. The Communication/Navigation Outage Forecasting System (C/NOFS) satellite, equipped with a suite of instruments, was launched into a low-inclination (13\({}^{\circ}\)) orbit with a perigee of 405 km and apogee of 853 km. C/NOFS data have yielded information not only about irregularity spectra [19; 24], but also about the zonal drift of plasma inside the EPBs [25].